Preprimitive streak and mesendoderm cells

ABSTRACT

This disclosure relates to compositions comprising human preprimitive streak cells and/or human mesendoderm cells as well as methods for their production. Additionally, disclosed herein are methods of identifying factors useful in the further differentiation of preprimitive streak and mesendoderm cell types.

RELATED APPLICATIONS

This application is a continuation of U.S. application No. 11/474,211,filed Jun. 23, 2006, now U.S. Pat. No. 7,985,585, which is acontinuation of PCT/US2005/024161 filed Jul. 8, 2005. PCT/US2005/024161claims the benefit of U.S. Provisional Application No. 60/693,364, filedJun. 23, 2005, U.S. Provisional Application No. 60/587,942, filed Jul.14, 2004, and U.S. Provisional Application No. 60/586,566, filed Jul. 9,2004. U.S. application No. 11/474,211 is also a continuation-in-part ofU.S. application No. 11/021,618 filed Dec. 23, 2004 (now U.S. Pat. No.7,510,876), which claims the benefit of U.S. Provisional Application No.60/587,942, filed Jul. 14, 2004, U.S. Provisional Application No.60/586,566, filed Jul. 9, 2004, and U.S. application No. 60/532,004,filed Dec. 23, 2003. U.S. application No. 11/474,211 also claims thebenefit of U.S. application No. 60/693,364, filed Jun. 23, 2005.

FIELD OF THE INVENTION

The present invention relates to the fields of medicine and cellbiology. In particular, the present invention relates to compositionscomprising preprimitive streak and/or mesendoderm cells as well asmethods of making, isolating and using such cells.

BACKGROUND

Human pluripotent stem cells, such as embryonic stem (ES) cells andembryonic germ (EG) cells, were first isolated in culture withoutfibroblast feeders in 1994 (Bongso et al. 1994) and with fibroblastfeeders (Hogan, 1997). Later, Thomson, Reubinoff and Shamblottestablished continuous cultures of human ES and EG cells usingmitotically inactivated mouse feeder layers (Reubinoff et al., 2000;Shamblott et al., 1998; Thomson et al., 1998).

Human ES and EG cells (hESCs) offer unique opportunities forinvestigating early stages of human development as well as fortherapeutic intervention in several disease states, such as diabetesmellitus and Parkinson's disease. For example, the use ofinsulin-producing β-cells derived from hESCs would offer a vastimprovement over current cell therapy procedures that utilize cells fromdonor pancreases for the treatment of diabetes. However, presently it isnot known how to generate an insulin-producing β-cell from hESCs. Assuch, current cell therapy treatments for diabetes mellitus, whichutilize islet cells from donor pancreases, are limited by the scarcityof high quality islet cells needed for transplant. Cell therapy for asingle Type I diabetic patient requires a transplant of approximately8×10⁸ pancreatic islet cells. (Shapiro et al., 2000; Shapiro et al.,2001a; Shapiro et al., 2001b). As such, at least two healthy donororgans are required to obtain sufficient islet cells for a successfultransplant. Human embryonic stem cells offer a source of startingmaterial from which to develop substantial quantities of high qualitydifferentiated cells for human cell therapies.

Two properties that make hESCs uniquely suited to cell therapyapplications are pluripotence and the ability to maintain these cells inculture for prolonged periods. Pluripotency is defined by the ability ofhESCs to differentiate to derivatives of all 3 primary germ layers(endoderm, mesoderm, ectoderm) which, in turn, form all somatic celltypes of the mature organism in addition to extraembryonic tissues (e.g.placenta) and germ cells. Although pluripotency imparts extraordinaryutility upon hESCs, this property also poses unique challenges for thestudy and manipulation of these cells and their derivatives. Owing tothe large variety of cell types that may arise in differentiating hESCcultures, the vast majority of cell types are produced at very lowefficiencies. Additionally, success in evaluating production of anygiven cell type depends critically on defining appropriate markers.Achieving efficient, directed differentiation is of great importance fortherapeutic application of hESCs.

As such, in addition to achieving efficient directed differentiation ofhESCs, it would be beneficial to identify markers which can be used toidentify and/or segregate cells at their earliest stages ofdifferentiation away from hESCs. Additionally, it would be beneficial toidentify factors which promote the differentiation of these earlyprecursor cells derived from hESCs to cell types useful for celltherapies.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to cell cultures comprisinghuman cells. In such cell cultures, at least about 5% of the human cellsare preprimitive streak cells, wherein the preprimitive streak cells aremultipotent cells that can differentiate into mesendoderm cells. Inother embodiments, at least about 10% to at least about 90% of the humancells in culture are preprimitive streak cells. In certain embodimentsdescribed herein, human feeder cells are also present in the cellcultures. In such embodiments, from at least about 5% to at least about75% of human cells other than feeder cells are preprimitive streakcells. In some embodiments, the preprimitive streak cells express amarker, such as FGF8 and/or nuclear-localized β-catenin. In certainembodiments, the expression of one or both of these markers is greaterthan the expression of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and/orSOX1. In still other embodiments described herein, the cell cultures aresubstantially free of visceral endodermal cells, parietal endodermalcells, primitive endodermal cells, definitive endodermal cells,ectodermal cells and/or mesodermal cells.

With respect to certain embodiments described herein, the preprimitivestreak cell cultures comprise pluripotent human cells, such as humanembryonic stem cells (hESCs). In such embodiments, from at least about 2to at least about 10 preprimitive streak cells are present for aboutevery 1 hESC in the cell cultures. In some embodiments the hESCs arederived from a morula, embryonic inner cell mass (ICM) or embryonicgonadal ridges. In some embodiments, the cell cultures containing humanpreprimitive streak cells comprise a medium comprising from less thanabout 2% (v/v) to from less than about 0.2% (v/v) serum. In preferredembodiments, such cell cultures comprise a medium that lacks serum orserum replacement. In other embodiments, the cell cultures containinghuman preprimitive streak cells comprise a growth factor of theNodal/Activin subgroup of the TGFβ superfamily. In preferredembodiments, the growth factor is activin A.

Additional embodiments described herein relate to cell culturescomprising mesendoderm cells, wherein the mesendoderm cells aremultipotent cells that can differentiate into mesoderm or definitiveendoderm cells. In such embodiments, the cell cultures comprise humancells, wherein at least about 5% of the human cells are mesendodermcells. In other embodiments, at least about 10% to at least about 90% ofthe human cells in culture are mesendoderm cells. In certain embodimentsdescribed herein, human feeder cells are also present in the cellcultures. In such embodiments, from at least about 5% to at least about75% of human cells other than feeder cells are mesendoderm cells. Insome embodiments, the mesendoderm cells express a marker, such asbrachyury, FGF4 and/or SNAI1. In certain embodiments, the expression ofone or more of these markers is greater than the expression of OCT4,SOX17, CXCR4, FOXA2, SOX7 and/or SOX1. In still other embodimentsdescribed herein, the cell cultures are substantially free of visceralendodermal cells, parietal endodermal cells, primitive endodermal cells,definitive endodermal cells, ectodermal cells and/or mesodermal cells.

With respect to certain embodiments described herein, the mesendodermcell cultures comprise pluripotent human cells, such as human embryonicstem cells (hESCs). In such embodiments, from at least about 2 to atleast about 10 mesendoderm cells are present for about every 1 hESC inthe cell cultures. In some embodiments the hESCs are derived from amorula, embryonic inner cell mass (ICM) or embryonic gonadal ridges. Insome embodiments, the cell cultures containing human mesendoderm cellscomprise a medium comprising from less than about 2% (v/v) to from lessthan about 0.2% (v/v) serum. In preferred embodiments, such cellcultures comprise a medium that lacks serum or serum replacement. Inother embodiments, the cell cultures containing human mesendoderm cellscomprise a growth factor of the Nodal/Activin subgroup of the TGFβsuperfamily. In preferred embodiments, the growth factor is activin A.

Further embodiments described herein relate to cell populationscomprising cells wherein at least about 90% of the cells are humanpreprimitive streak cells. In these embodiments, the preprimitive streakcells are multipotent cells that can differentiate into mesendodermcells. In other embodiments, at least about 95% to at least about 98% ofthe human cells in the population are preprimitive streak cells. In someembodiments, preprimitive streak cells express a marker, such as FGF8and/or nuclear-localized β-catenin. In certain embodiments, theexpression of one or both of these markers is greater than theexpression of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and/or SOX1. Instill other embodiments described herein, the cell populations aresubstantially free of visceral endodermal cells, parietal endodermalcells, primitive endodermal cells, definitive endodermal cells,ectodermal cells and/or mesodermal cells.

Still other embodiments described herein relate to cell populationscomprising cells wherein at least about 90% of the cells are humanmesendoderm cells. In these embodiments, the mesendoderm cells aremultipotent cells that can differentiate into mesoderm cells and/ordefinitive endoderm cells. In other embodiments, at least about 95% toat least about 98% of the human cells in the population are mesendodermcells. In some embodiments, mesendoderm cells express a marker, such asbrachyury, FGF4 and/or SNAI1. In certain embodiments, the expression ofone or both of these markers is greater than the expression of OCT4,SOX17, CXCR4, FOXA2, SOX7 and/or SOX1. In still other embodimentsdescribed herein, the cell populations are substantially free ofvisceral endodermal cells, parietal endodermal cells, primitiveendodermal cells, definitive endodermal cells, ectodermal cells and/ormesodermal cells.

Additional embodiments described herein relate to methods of producingpreprimitive streak cells. In such methods, a cell population comprisingpluripotent human cells, such as hESCs, is obtained. Pluripotent humancells within the cell population are differentiated in a mediumcomprising less than about 2% serum and at least one growth factor ofthe TGFβ superfamily, wherein the growth factor is present in the mediumin an amount sufficient to promote differentiation of at least a portionof said pluripotent cells to preprimitive streak cells which aremultipotent and can differentiate into mesendoderm cells. Someembodiments include a further step that comprises allowing sufficienttime for preprimitive streak cells to form, wherein said sufficient timefor preprimitive streak cells to form has been determined by detectingthe presence of preprimitive streak cells in said cell population. Insome embodiments, sufficient time is at least about 6 hours. In otherembodiments, detecting the presence of preprimitive streak cells in thecell population comprises detecting the expression of at least onemarker selected from the group consisting of FGF8 and nuclear-localizedβ-catenin and at least one marker from the group consisting ofbrachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1 in cells of the cellpopulation, wherein the expression of a marker selected from the groupconsisting of FGF8 and nuclear-localized β-catenin is greater than theexpression of a marker selected from the group consisting of brachyury,FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1 in said preprimitive streakcells. In such embodiments, marker detection can be by quantitativepolymerase chain reaction (Q-PCR), immunocytochemistry or othercomparable method.

With respect to certain methods described herein, from at least about 5%to at least about 90% of the human cells in culture differentiate intopreprimitive streak cells. In some embodiments of the methods describedherein, the growth factor present in the medium is a growth factor ofthe Nodal/Activin subgroup of the TGFβ superfamily. In preferredembodiments, the growth factor is activin A. In other preferredembodiments, the growth factor is present in the medium at aconcentration ranging from at least about 10 ng/ml to at least about1000 ng/ml. In certain embodiments, the growth factor is withdrawn afterabout 6 hours, 12 hours or 18 hours. In additional embodiments, themedium comprises from less than about 1% (v/v) to less than about 0.2%(v/v) serum. In other embodiments, the medium is low serum RPMI. Instill other embodiments, the cell population is differentiated in theabsence of serum or serum replacement.

Other methods described herein are methods of producing mesendodermcells. In such methods, a cell population comprising pluripotent humancells, such as hESCs, is obtained. Pluripotent human cells within thecell population are differentiated in a medium comprising less thanabout 2% serum and at least one growth factor of the TGFβ superfamily,wherein the growth factor is present in the medium in an amountsufficient to promote differentiation of at least a portion of saidpluripotent cells to mesendoderm cells which are multipotent and candifferentiate into mesoderm cells and/or definitive endoderm cells. Someembodiments include a further step that comprises allowing sufficienttime for mesendoderm cells to form, wherein said sufficient time formesendoderm cells to form has been determined by detecting the presenceof mesendoderm cells in said cell population. In some embodiments,sufficient time is at least about 24 hours. In other embodiments,detecting the presence of mesendoderm in the cell population comprisesdetecting the expression of at least one marker selected from the groupconsisting of brachyury, FGF4 and/or SNAI1 and at least one marker fromthe group consisting of OCT4, SOX17, CXCR4, FOXA2, SOX7 and/or SOX1 incells of the cell population, wherein the expression of a markerselected from the group consisting of brachyury, FGF4 and/or SNAI1 isgreater than the expression of a marker selected from the groupconsisting of OCT4, SOX17, CXCR4, FOXA2, SOX7 and/or SOX1 in saidmesendoderm cells. In such embodiments, marker detection can be byquantitative polymerase chain reaction (Q-PCR), immunocytochemistry orother comparable method.

With respect to certain methods described herein, from at least about 5%to at least about 90% of the human cells in culture differentiate intomesendoderm cells. In some embodiments of the methods described herein,the growth factor present in the medium is a growth factor of theNodal/Activin subgroup of the TGFβ superfamily. In preferredembodiments, the growth factor is activin A. In other preferredembodiments, the growth factor is present in the medium at aconcentration ranging from at least about 10 ng/ml to at least about1000 ng/ml. In certain embodiments, the growth factor is withdrawn afterabout 24 hours, 36 hours or 48 hours. In additional embodiments, themedium comprises from less than about 1% (v/v) to less than about 0.2%(v/v) serum. In other embodiments, the medium is low serum RPMI. Instill other embodiments, the cell population is differentiated in theabsence of serum or serum replacement.

Still other embodiments described herein relate to methods for producinga cell population that is enriched in preprimitive streak cells. Suchmethods comprise the steps of (a) obtaining a population of pluripotentcells, such as hESCs, wherein at least a one cell of the pluripotentcell population comprises a copy of a nucleic acid sequence encodinggreen fluorescent protein (GFP) or a biologically active fragmentthereof under the control of the FGF8 promoter, (b) differentiating thepluripotent cells so as to produce preprimitive streak cells which aremultipotent cells that can differentiate into mesendoderm cells, and (c)separating the preprimitive streak cells from cells that do not expressGFP. In some embodiments, the cell population comprises at least about95% to at least about 98% preprimitive streak cells. In someembodiments, the differentiating step of the methods described hereincomprises providing a pluripotent cell population with at least onegrowth factor of the TGFβ superfamily, such as activin A. Preferredconcentrations of activin A range from at least about 50 ng/ml to atleast about 500 ng/ml. In additional embodiments, the cell population isdifferentiated in a medium comprising from less than about 1% (v/v) toless than about 0.1% (v/v) serum. In other embodiments, the medium islow serum RPMI. In still other embodiments, the cell population isdifferentiated in the absence of serum or serum replacement.

Other embodiments described herein relate to methods for producing acell population that is enriched in mesendoderm cells. Such methodscomprise the steps of (a) obtaining a populations of pluripotent cells,such as hESCs, wherein at least a one cell of the pluripotent cellpopulation comprises a copy of a nucleic acid sequence encoding greenfluorescent protein (GFP) or a biologically active fragment thereofunder the control of the brachyury, FGF4 or SNAI1 promoter, (b)differentiating the pluripotent cells so as to produce mesendoderm cellswhich are multipotent cells that can differentiate into mesoderm cellsand/or definitive endoderm cells, and (c) separating the mesendodermcells from cells that do not express GFP. In some embodiments, the cellpopulation comprises at least about 95% to at least about 98%mesendoderm cells. In some embodiments, the differentiating step of themethods described herein comprises providing a pluripotent cellpopulation with at least one growth factor of the TGFβ superfamily, suchas activin A. Preferred concentrations of activin A range from at leastabout 50 ng/ml to at least about 500 ng/ml. In additional embodiments,the cell population is differentiated in a medium comprising from lessthan about 1% (v/v) to less than about 0.1% (v/v) serum. In otherembodiments, the medium is low serum RPMI. In still other embodiments,the cell population is differentiated in the absence of serum or serumreplacement.

Some embodiments described herein are screening methods for identifyinga differentiation factor capable of promoting the differentiation ofpreprimitive streak cells in a cell population comprising human cells.Such methods comprise the steps of (a) obtaining a cell populationcomprising human preprimitive streak cells, (b) providing a candidatedifferentiation factor to the cell population, (c) determiningexpression of a marker in the cell population at a first time point,determining expression of the same marker in the cell population at asecond time point, wherein the second time point is subsequent to thefirst time point and wherein the second time point is subsequent toproviding the population with the candidate differentiation factor, (d)and determining if expression of the marker in the cell population atthe second time point is increased or decreased as compared to theexpression of the marker in the cell population at the first time point,wherein an increase or decrease in expression of the marker in the cellpopulation indicates that the candidate differentiation factor iscapable of promoting the differentiation of the preprimitive streakcells. In certain embodiments, the first time point is prior to or atapproximately the same time as providing the candidate differentiationfactor. In other embodiments, the first time point is subsequent toproviding the candidate differentiation factor. In some embodiments ofthe screening methods described herein, the human preprimitive streakcells differentiate into cells, such as mesendoderm cells, mesodermcells and/or definitive endoderm cells, in response to the candidatedifferentiation factor. In some embodiments, mesendoderm is indicated bythe expression of markers, such as brachyury, FGF4 and/or SNAI1. Inother embodiments, mesoderm is indicated by the expression of markers,such as FOXF1, FLK1, BMP4, MOX1 and SDF1. In still other embodiments,definitive endoderm is indicated by the expression of markers, such asCXCR4 and/or SOX17.

With respect to the screening methods described herein, certainembodiments relate to providing a candidate differentiation factor, suchas at least one growth factor from the TGFβ superfamily, such as activinA. In other embodiments, the candidate differentiation factor is a smallmolecule or a polypeptide. In still other embodiments, the candidatedifferentiation factor is not a factor of the TGFβ superfamily. In stillother embodiments, the candidate differentiation factor is a factor thatis not known to cause the differentiation of preprimitive streak cells.

Other embodiments described herein are screening methods for identifyinga differentiation factor capable of promoting the differentiation ofmesendoderm cells in a cell population comprising human cells. Suchmethods comprise the steps of (a) obtaining a cell population comprisinghuman mesendoderm cells, (b) providing a candidate differentiationfactor to the cell population, (c) determining expression of a marker inthe cell population at a first time point, determining expression of thesame marker in the cell population at a second time point, wherein thesecond time point is subsequent to the first time point and wherein thesecond time point is subsequent to providing the population with thecandidate differentiation factor, (d) and determining if expression ofthe marker in the cell population at the second time point is increasedor decreased as compared to the expression of the marker in the cellpopulation at the first time point, wherein an increase or decrease inexpression of the marker in the cell population indicates that thecandidate differentiation factor is capable of promoting thedifferentiation of the mesendoderm cells. In certain embodiments, thefirst time point is prior to or at approximately the same time asproviding the candidate differentiation factor. In other embodiments,the first time point is subsequent to providing the candidatedifferentiation factor. In some embodiments of the screening methodsdescribed herein, the human mesendoderm cells differentiate into cells,such as mesoderm cells and/or definitive endoderm cells, in response tothe candidate differentiation factor. In some embodiments, mesoderm isindicated by the expression of markers, such as FOXF1, FLK1, BMP4, MOX1and SDF1. In other embodiments, definitive endoderm is indicated by theexpression of markers, such as CXCR4 and/or SOX17.

With respect to the screening methods described herein, certainembodiments relate to providing a candidate differentiation factor, suchas at least one growth factor from the TGFβ superfamily, such as activinA. In other embodiments, the candidate differentiation factor is a smallmolecule or a polypeptide. In still other embodiments, the candidatedifferentiation factor is not a factor of the TGFβ superfamily. In stillother embodiments, the candidate differentiation factor is a factor thatis not known to cause the differentiation of mesendoderm cells.

Additional embodiments relate to a method of increasing the expressionof the FGF8 gene product in a human embryonic stem cell (hESC) in vitro.The method comprises obtaining an hESC in a medium comprising less thanabout 2% (v/v) serum and contacting the hESC with a differentiationfactor in an amount sufficient to increase expression of the FGF8 geneproduct. In some embodiments, the differentiation factor is at least onegrowth factor from the TGFβ superfamily, such as activin A. In someembodiments, the medium does not comprise serum replacement.

Still other embodiments relate to a method of increasing the expressionof a gene product selected from the group consisting of brachyury, FGF4and SNAI1 in a human embryonic stem cell (hESC) in vitro. The methodcomprises obtaining an hESC in a medium comprising less than about 2%(v/v) serum and contacting the hESC with a differentiation factor in anamount sufficient to increase expression of a gene product selected fromthe group consisting of brachyury, FGF4 and SNAI1. In some embodiments,the differentiation factor is at least one growth factor from the TGFβsuperfamily, such as activin A. In some embodiments, the medium does notcomprise serum replacement.

Some embodiments described herein relate to a cell culture comprisinghuman embryonic stem cells (hESCs) and a medium comprising less thanabout 2% (v/v) serum, wherein the hESCs begin differentiating at areference time point such that expression of FGF8 mRNA is substantiallyupregulated as compared to baseline FGF8 mRNA expression in the hESCs byabout 6 hours from the reference time point. In some embodiments, theexpression of β-catenin polypeptide begins to become localized to thecell nucleus by about 17 hours from the reference time point. In otherembodiments, the expression of brachyury, FGF4 and/or SNAI1 mRNA issubstantially upregulated by about 24 hours from the reference timepoint. In still other embodiments, the expression of E-cadherin mRNAbegins to be downregulated by about 12 hours from the reference timepoint. Additionally, in some embodiments, the expression of SOX17 mRNAis substantially upregulated by about 48 hours from the reference timepoint and/or the expression of FOXA2 mRNA is substantially upregulatedby about 96 hours from the reference time point. In certain embodimentsof the cell cultures described herein, the medium comprises from lessthan about 1% (v/v) to less than about 0.2% (v/v) serum. In otherembodiments, the medium comprises about 0% (v/v) serum. In still otherembodiments, the medium does not comprise serum replacement.

Additional embodiments described herein relate to cells culturecomprising human embryonic stem cells, a differentiation factor of theTGFβ superfamily and a medium comprising less than about 2% (v/v) serum,wherein a first set of marker genes is upregulated or downregulatedprior to, or at about the same time as, the upregulation or peakexpression of a second set and/or a third set of marker genes. In someembodiments the medium does not include serum or serum replacement.

Still other embodiments relate to methods of differentiating cells in acell culture by contacting a cell culture comprising human embryonicstem cells with a medium comprising less that about 2% serum, providingthe hESCs with a differentiation factor of the TGFβ superfamily, andpermitting differentiation of the hESCs to occur. In some embodiments,such methods produce cells having a first set of marker genes that isupregulated or downregulated prior to, or at about the same time as, theupregulation or peak expression of a second set and/or a third set ofmarker genes. In some embodiments the medium does not include serum orserum replacement.

In certain jurisdictions, there may not be any generally accepteddefinition of the term “comprising.” As used herein, the term“comprising” is intended to represent “open” language which permits theinclusion of any additional elements. With this in mind, additionalembodiments of the present inventions are described with reference tothe numbered paragraphs below:

1. A cell culture comprising human cells wherein at least about 5% ofsaid human cells are preprimitive streak cells, said preprimitive streakcells being multipotent cells that can differentiate into mesendodermcells.

2. The cell culture of paragraph 1, wherein at least about 10% of saidhuman cells are preprimitive streak cells.

3. The cell culture of paragraph 1, wherein at least about 20% of saidhuman cells are preprimitive streak cells.

4. The cell culture of paragraph 1, wherein at least about 30% of saidhuman cells are preprimitive streak cells.

5. The cell culture of paragraph 1, wherein at least about 40% of saidhuman cells are preprimitive streak cells.

6. The cell culture of paragraph 1, wherein at least about 50% of saidhuman cells are preprimitive streak cells.

7. The cell culture of paragraph 1, wherein at least about 60% of saidhuman cells are preprimitive streak cells.

8. The cell culture of paragraph 1, wherein at least about 70% of saidhuman cells are preprimitive streak cells.

9. The cell culture of paragraph 1, wherein at least about 80% of saidhuman cells are preprimitive streak cells.

10. The cell culture of paragraph 1, wherein at least about 90% of saidhuman cells are preprimitive streak cells.

11. The cell culture of paragraph 1, wherein human feeder cells arepresent in said culture, and wherein at least about 5% of human cellsother than said human feeder cells are preprimitive streak cells.

12. The cell culture of paragraph 1, wherein human feeder cells arepresent in said culture, and wherein at least about 25% of human cellsother than said human feeder cells are preprimitive streak cells.

13. The cell culture of paragraph 1, wherein human feeder cells arepresent in said culture, and wherein at least about 50% of human cellsother than said human feeder cells are preprimitive streak cells.

14. The cell culture of paragraph 1, wherein human feeder cells arepresent in said culture, and wherein at least about 75% of human cellsother than said human feeder cells are preprimitive streak cells.

15. The cell culture of paragraph 1, wherein said primitive streak cellsexpress a marker selected from the group consisting of FGF8 andnuclear-localized β-catenin.

16. The cell culture of paragraph 15, wherein the expression of a markerselected from the group consisting of FGF8 and nuclear-localizedβ-catenin is greater than the expression of a marker selected from thegroup consisting of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1in said preprimitive streak cells.

17. The cell culture of paragraph 15, wherein said preprimitive streakcells do not substantially express a marker selected from the groupconsisting of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1.

18. The cell culture of paragraph 1, wherein said preprimitive streakcells express FGF8 and nuclear-localized β-catenin.

19. The cell culture of paragraph 18, wherein the expression of FGF8 andnuclear-localized β-catenin is greater than the expression of brachyury,FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1 in said preprimitive streakcells.

20. The cell culture of paragraph 18, wherein said preprimitive streakcells do not substantially express brachyury, FGF4, SNAI1, SOX17, FOXA2,SOX7 and SOX1.

21. The cell culture of paragraph 1, wherein said cell culture issubstantially free of cells selected from the group consisting ofvisceral endodermal cells, parietal endodermal cells, primitiveendodermal cells, definitive endodermal cells, ectodermal cells andmesodermal cells.

22. The cell culture of paragraph 1 further comprising human embryonicstem cells (hESCs).

23. The cell culture of paragraph 22, wherein at least about 2preprimitive streak cells are present for about every 1 hESC in saidcell culture.

24. The cell culture of paragraph 22, wherein at least about 10preprimitive streak cells are present for about every 1 hESC in saidcell culture.

25. The cell culture of paragraph 22, wherein said hESC is derived froma tissue selected from the group consisting of the morula, the innercell mass (ICM) of an embryo and the gonadal ridges of an embryo.

26. The cell culture of paragraph 1 further comprising a medium thatcomprises less than about 2% (v/v) serum.

27. The cell culture of paragraph 1 further comprising a medium thatcomprises less than about 1% (v/v) serum.

28. The cell culture of paragraph 1 further comprising a medium thatcomprises less than about 0.5% (v/v) serum.

29. The cell culture of paragraph 1 further comprising a medium thatcomprises less than about 0.2% (v/v) serum.

30. The cell culture of paragraph 1 further comprising a medium thatlacks serum or lacks serum replacement.

31. The cell culture of paragraph 1 further comprising a growth factorof the Nodal/Activin subgroup of the TGFβ superfamily.

32. The cell culture of paragraph 31, wherein said growth factor of theNodal/Activin subgroup of the TGFβ superfamily comprises activin A.

33. A cell culture comprising human cells wherein at least about 5% ofsaid human cells are mesendoderm cells, said mesendoderm cells beingmultipotent cells that can differentiate into mesoderm cells ordefinitive endoderm cells.

34. The cell culture of paragraph 33, wherein at least about 10% of saidhuman cells are mesendoderm cells.

35. The cell culture of paragraph 33, wherein at least about 20% of saidhuman cells are mesendoderm cells.

36. The cell culture of paragraph 33, wherein at least about 30% of saidhuman cells are mesendoderm cells.

37. The cell culture of paragraph 33, wherein at least about 40% of saidhuman cells are preprimitive streak cells.

38. The cell culture of paragraph 33, wherein at least about 50% of saidhuman cells are mesendoderm cells.

39. The cell culture of paragraph 33, wherein at least about 60% of saidhuman cells are mesendoderm cells.

40. The cell culture of paragraph 33, wherein at least about 70% of saidhuman cells are mesendoderm cells.

41. The cell culture of paragraph 33, wherein at least about 80% of saidhuman cells are mesendoderm cells.

42. The cell culture of paragraph 33, wherein at least about 90% of saidhuman cells are mesendoderm cells.

43. The cell culture of paragraph 33, wherein human feeder cells arepresent in said culture, and wherein at least about 5% of human cellsother than said human feeder cells are mesendoderm cells.

44. The cell culture of paragraph 33, wherein human feeder cells arepresent in said culture, and wherein at least about 25% of human cellsother than said human feeder cells are mesendoderm cells.

45. The cell culture of paragraph 33, wherein human feeder cells arepresent in said culture, and wherein at least about 50% of human cellsother than said human feeder cells are mesendoderm cells.

46. The cell culture of paragraph 33, wherein human feeder cells arepresent in said culture, and wherein at least about 75% of human cellsother than said human feeder cells are mesendoderm cells.

47. The cell culture of paragraph 33, wherein said mesendoderm cellsexpress a marker selected from the group consisting of brachyury, FGF4and SNAI1.

48. The cell culture of paragraph 47, wherein the expression of a markerselected from the group consisting of brachyury, FGF4 and SNAI1 isgreater than the expression of a marker selected from the groupconsisting of OCT4, SOX17, CXCR4, FOXA2, SOX7 and SOX1 in saidmesendoderm cells.

49. The cell culture of paragraph 47, wherein said mesendoderm cells donot substantially express a marker selected from the group consisting ofOCT4, SOX17, CXCR4, FOXA2, SOX7 and SOX1.

50. The cell culture of paragraph 33, wherein said mesendoderm cellsexpress brachyury, FGF4 and SNAI1.

51. The cell culture of paragraph 50, wherein the expression ofbrachyury, FGF4 and SNAI1 is greater than the expression of OCT4, SOX17,CXCR4, FOXA2, SOX7 and SOX1 in said mesendoderm cells.

52. The cell culture of paragraph 51, wherein said mesendoderm cells donot substantially express OCT4, SOX17, CXCR4, FOXA2, SOX7 and SOX1.

53. The cell culture of paragraph 33, wherein said cell culture issubstantially free of cells selected from the group consisting ofvisceral endodermal cells, parietal endodermal cells, primitiveendodermal cells, definitive endodermal cells, ectodermal cells andmesodermal cells.

54. The cell culture of paragraph 33 further comprising human embryonicstem cells (hESCs).

55. The cell culture of paragraph 54, wherein at least about 2mesendoderm cells are present for about every 1 hESC in said cellculture.

56. The cell culture of paragraph 54, wherein at least about 10mesendoderm cells are present for about every 1 hESC in said cellculture.

57. The cell culture of paragraph 54, wherein said hESC is derived froma tissue selected from the group consisting of the morula, the innercell mass (ICM) of an embryo and the gonadal ridges of an embryo.

58. The cell culture of paragraph 33 further comprising a medium thatcomprises less than about 2% (v/v) serum.

59. The cell culture of paragraph 33 further comprising a medium thatcomprises less than about 1% (v/v) serum.

60. The cell culture of paragraph 33 further comprising a medium thatcomprises less than about 0.5% (v/v) serum.

61. The cell culture of paragraph 33 further comprising a medium thatcomprises less than about 0.2% (v/v) serum.

62. The cell culture of paragraph 33 further comprising a medium thatlacks serum or lacks serum replacement.

63. The cell culture of paragraph 33 further comprising a growth factorof the Nodal/Activin subgroup of the TGFβ superfamily.

64. The cell culture of paragraph 63, wherein said growth factor of theNodal/Activin subgroup of the TGFβ superfamily comprises activin A.

65. A cell population comprising cells wherein at least about 90% ofsaid cells are human preprimitive streak cells, said preprimitive streakcells being multipotent cells that can differentiate into mesendodermcells.

66. The cell population of paragraph 65, wherein at least about 95% ofsaid cells are human preprimitive streak cells.

67. The cell population of paragraph 65, wherein at least about 98% ofsaid cells are human preprimitive streak cells.

68. The cell population of paragraph 65, wherein said human preprimitivestreak cells express a marker selected from the group consisting of FGF8and nuclear-localized β-catenin.

69. The cell population of paragraph 68, wherein the expression of amarker selected from the group consisting of FGF8 and nuclear-localizedβ-catenin is greater than the expression of a marker selected from thegroup consisting of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1in said human preprimitive streak cells.

70. The cell population of paragraph 68, wherein said human preprimitivestreak cells do not substantially express a marker selected from thegroup consisting of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1.

71. The cell population of paragraph 65, wherein said human preprimitivestreak cells express FGF8 and nuclear-localized β-catenin.

72. The cell population of paragraph 71, wherein the expression of FGF8and β-catenin is greater than the expression of brachyury, FGF4, SNAI1,SOX17, FOXA2, SOX7 and SOX1 in said human preprimitive streak cells.

73. The cell population of paragraph 71, wherein said human preprimitivestreak cells do not substantially express brachyury, FGF4, SNAI1, SOX17,FOXA2, SOX7 and SOX1.

74. A cell population comprising cells wherein at least about 90% ofsaid cells are human mesendoderm cells, said mesendoderm cells beingmultipotent cells that can differentiate into mesoderm cells ordefinitive endoderm cells.

75. The cell population of paragraph 74, wherein at least about 95% ofsaid cells are human mesendoderm cells.

76. The cell population of paragraph 74, wherein at least about 98% ofsaid cells are human mesendoderm cells.

77. The cell population of paragraph 74, wherein said human mesendodermcells express a marker selected from the group consisting of brachyury,FGF4 and SNAI1.

78. The cell population of paragraph 77, wherein the expression of amarker selected from the group consisting of brachyury, FGF4 and SNAI1is greater than the expression of a marker selected from the groupconsisting of OCT4, SOX17, CXCR4, FOXA2, SOX7 and SOX1 in said humanmesendoderm cells.

79. The cell population of paragraph 77, wherein said human mesendodermcells do not substantially express a marker selected from the groupconsisting of OCT4, SOX17, CXCR4, FOXA2, SOX7 and SOX1.

80. The cell population of paragraph 74, wherein said human mesendodermcells express brachyury, FGF4 and SNAI1.

81. The cell population of paragraph 80, wherein the expression ofbrachyury, FGF4 and SNAI1 is greater than the expression of OCT4, SOX17,CXCR4, FOXA2, SOX7 and SOX1 in said human mesendoderm cells.

82. The cell population of paragraph 80, wherein said human mesendodermcells do not substantially express OCT4, SOX17, CXCR4, FOXA2, SOX7 andSOX1.

83. A method of producing preprimitive streak cells, said methodcomprising the steps of obtaining a cell population comprisingpluripotent human cells and differentiating said pluripotent human cellsin a medium comprising less than about 2% serum and at least one growthfactor of the TGFβ superfamily, wherein said growth factor is present inthe medium in an amount sufficient to promote differentiation of atleast a portion of said pluripotent cells to preprimitive streak cells,said preprimitive streak cells being multipotent cells that candifferentiate into mesendoderm cells.

84. The method of paragraph 83 further comprising the step of allowingsufficient time for preprimitive streak cells to form, wherein saidsufficient time for preprimitive streak cells to form has beendetermined by detecting the presence of preprimitive streak cells insaid cell population.

85. The method of paragraph 84, wherein sufficient time is at leastabout 6 hours.

86. The method of paragraph 84, wherein detecting the presence ofpreprimitive streak cells in said cell population comprises detectingthe expression of at least one marker selected from the group consistingof FGF8 and nuclear-localized β-catenin and at least one marker from thegroup consisting of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1in cells of said cell population, wherein the expression of a markerselected from the group consisting of FGF8 and nuclear-localizedβ-catenin is greater than the expression of a marker selected from thegroup consisting of brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and SOX1in said preprimitive streak cells.

87. The method of paragraph 86, wherein the expression of at least oneof said markers is determined by quantitative polymerase chain reaction(Q-PCR).

88. The method of paragraph 86, wherein the expression of at least oneof said markers is determined by immunocytochemistry.

89. The method of paragraph 83, wherein at least about 5% of saidpluripotent human cells differentiate into preprimitive streak cells.

90. The method of paragraph 83, wherein at least about 10% of saidpluripotent human cells differentiate into preprimitive streak cells.

91. The method of paragraph 83, wherein at least about 20% of saidpluripotent human cells differentiate into preprimitive streak cells.

92. The method of paragraph 83, wherein at least about 30% of saidpluripotent human cells differentiate into preprimitive streak cells.

93. The method of paragraph 83, wherein at least about 40% of saidpluripotent human cells differentiate into preprimitive streak cells.

94. The method of paragraph 83, wherein at least about 50% of saidpluripotent human cells differentiate into preprimitive streak cells.

95. The method of paragraph 83, wherein at least about 60% of saidpluripotent human cells differentiate into preprimitive streak cells.

96. The method of paragraph 83, wherein at least about 70% of saidpluripotent human cells differentiate into preprimitive streak cells.

97. The method of paragraph 83, wherein at least about 80% of saidpluripotent human cells differentiate into preprimitive streak cells.

98. The method of paragraph 83, wherein at least about 90% of saidpluripotent human cells differentiate into preprimitive streak cells.

99. The method of paragraph 83, wherein said at least one growth factoris of the Nodal/Activin subgroup of the TGFβ superfamily.

100. The method of paragraph 99, wherein said at least one growth factorof the Nodal/Activin subgroup of the TGFβ superfamily comprises activinA.

101. The method of paragraph 83, wherein said at least one growth factoris provided in a concentration of at least about 10 ng/ml.

102. The method of paragraph 83, wherein said at least one growth factoris provided in a concentration of at least about 100 ng/ml.

103. The method of paragraph 83, wherein said at least one growth factoris provided in a concentration of at least about 500 ng/ml.

104. The method of paragraph 83, wherein said at least one growth factoris provided in a concentration of at least about 1000 ng/ml.

105. The method of paragraph 83, wherein said at least one growth factoris withdrawn after about 6 hours.

106. The method of paragraph 83, wherein said at least one growth factoris withdrawn after about 12 hours.

107. The method of paragraph 83, wherein said at least one growth factoris withdrawn after about 18 hours.

108. The method of paragraph 83, wherein said cell population isdifferentiated in a medium comprising less than about 1% (v/v) serum.

109. The method of paragraph 83, wherein said cell population isdifferentiated in a medium comprising less than about 0.5% (v/v) serum.

110. The method of paragraph 83, wherein said cell population isdifferentiated in a medium comprising less than about 0.2% (v/v) serum.

111. The method of paragraph 83, wherein said cell population isdifferentiated in a medium comprising less than about 0.1% (v/v) serum.

112. The method of paragraph 83, wherein said cell population isdifferentiated in the absence of serum or the absence of serumreplacement.

113. The method of paragraph 83, wherein said cell population isdifferentiated in a medium comprising about 0% (v/v) serum on about thefirst day after adding said at least one growth factor, about 0.2% (v/v)serum or less on about the second day after adding said at least onegrowth factor and about 2% (v/v) serum or less on about the third dayafter adding said at least one growth factor.

114. The method of paragraph 83, wherein said cell population isdifferentiated in low serum RPMI medium.

115. The method of paragraph 83, wherein said pluripotent human cellscomprise human embryonic stem cells (hESCs).

116. The method of paragraph 115, wherein said human embryonic stemcells are derived from a tissue selected from the group consisting ofthe morula, the inner cell mass (ICM) of an embryo and the gonadalridges of an embryo.

117. A preprimitive streak cell produced by the method of paragraph 83.

118. A method of producing mesendoderm cells, said method comprising thesteps of obtaining a cell population comprising pluripotent human cellsand differentiating said pluripotent human cells in a medium comprisingless than about 2% serum and at least one growth factor of the TGFβsuperfamily, wherein said growth factor is present in the medium in anamount sufficient to promote differentiation of at least a portion ofsaid pluripotent cells to mesendoderm cells, said mesendoderm cellsbeing multipotent cells that can differentiate into mesendoderm cells.

119. The method of paragraph 118 further comprising the step of allowingsufficient time for mesendoderm cells to form, wherein said sufficienttime for mesendoderm cells to form has been determined by detecting thepresence of mesendoderm cells in said cell population.

120. The method of paragraph 119, wherein sufficient time is at leastabout 24 hours.

121. The method of paragraph 118, wherein detecting the presence ofmesendoderm cells in said cell population comprises detecting theexpression of at least one marker selected from the group consisting ofbrachyury, FGF4 and SNAI1 and at least one marker from the groupconsisting of OCT4, SOX17, CXCR4, FOXA2, SOX7 and SOX1 in cells of saidcell population, wherein the expression of a marker selected from thegroup consisting of brachyury, FGF4 and SNAI1 is greater than theexpression of a marker selected from the group consisting of OCT4,SOX17, CXCR4, FOXA2, SOX7 and SOX1 in said mesendoderm cells.

122. The method of paragraph 121, wherein the expression of at least oneof said markers is determined by quantitative polymerase chain reaction(Q-PCR).

123. The method of paragraph 121, wherein the expression of at least oneof said markers is determined by immunocytochemistry.

124. The method of paragraph 118, wherein at least about 5% of saidpluripotent human cells differentiate into mesendoderm cells.

125. The method of paragraph 118, wherein at least about 10% of saidpluripotent human cells differentiate into mesendoderm cells.

126. The method of paragraph 118, wherein at least about 20% of saidpluripotent human cells differentiate into mesendoderm cells.

127. The method of paragraph 118, wherein at least about 30% of saidpluripotent human cells differentiate into mesendoderm cells.

128. The method of paragraph 118, wherein at least about 40% of saidpluripotent human cells differentiate into mesendoderm cells.

129. The method of paragraph 118, wherein at least about 50% of saidpluripotent human cells differentiate into mesendoderm cells.

130. The method of paragraph 118, wherein at least about 60% of saidpluripotent human cells differentiate into mesendoderm cells.

131. The method of paragraph 118, wherein at least about 70% of saidpluripotent human cells differentiate into mesendoderm cells.

132. The method of paragraph 118, wherein at least about 80% of saidpluripotent human cells differentiate into mesendoderm cells.

133. The method of paragraph 118, wherein at least about 90% of saidpluripotent human cells differentiate into mesendoderm cells.

134. The method of paragraph 118, wherein said at least one growthfactor is of the Nodal/Activin subgroup of the TGFβ superfamily.

135. The method of paragraph 134, wherein said at least one growthfactor of the Nodal/Activin subgroup of the TGFβ superfamily comprisesactivin A.

136. The method of paragraph 118, wherein said at least one growthfactor is provided in a concentration of at least about 10 ng/ml.

137. The method of paragraph 118, wherein said at least one growthfactor is provided in a concentration of at least about 100 ng/ml.

138. The method of paragraph 118, wherein said at least one growthfactor is provided in a concentration of at least about 500 ng/ml.

139. The method of paragraph 118, wherein said at least one growthfactor is provided in a concentration of at least about 1000 ng/ml.

140. The method of paragraph 118, wherein said at least one growthfactor is withdrawn after about 24 hours.

141. The method of paragraph 118, wherein said at least one growthfactor is withdrawn after about 36 hours.

142. The method of paragraph 118, wherein said at least one growthfactor is withdrawn after about 48 hours.

143. The method of paragraph 118, wherein said cell population isdifferentiated in a medium comprising less than about 1% (v/v) serum.

144. The method of paragraph 118, wherein said cell population isdifferentiated in a medium comprising less than about 0.5% (v/v) serum.

145. The method of paragraph 118, wherein said cell population isdifferentiated in a medium comprising less than about 0.2% (v/v) serum.

146. The method of paragraph 118, wherein said cell population isdifferentiated in a medium comprising less than about 0.1% (v/v) serum.

147. The method of paragraph 118, wherein said cell population isdifferentiated in the absence of serum or the absence of serumreplacement.

148. The method of paragraph 118, wherein said cell population isdifferentiated in a medium comprising about 0% (v/v) serum on about thefirst day after adding said at least one growth factor, about 0.2% (v/v)serum or less on about the second day after adding said at least onegrowth factor and about 2% (v/v) serum or less on about the third dayafter adding said at least one growth factor.

149. The method of paragraph 118, wherein said cell population isdifferentiated in low serum RPMI medium.

150. The method of paragraph 118, wherein said pluripotent human cellscomprise human embryonic stem cells (hESCs).

151. The method of paragraph 150, wherein said human embryonic stemcells are derived from a tissue selected from the group consisting ofthe morula, the inner cell mass (ICM) of an embryo and the gonadalridges of an embryo.

152. A mesendoderm cell produced by the method of paragraph 118.

153. A method of producing a cell population enriched in preprimitivestreak cells, said method comprising the steps of obtaining a populationof pluripotent cells, wherein at least one cell of said pluripotent cellpopulation comprises at least one copy of a nucleic acid under thecontrol of the FGF8 promoter, said nucleic acid comprising a sequenceencoding green fluorescent protein (GFP) or a biologically activefragment thereof; differentiating said pluripotent cells so as toproduce preprimitive streak cells, said preprimitive streak cells beingmultipotent cells that can differentiate into mesendoderm cells, andseparating said preprimitive streak cells from cells which do notexpress GFP.

154. The method of paragraph 153, wherein said enriched cell populationcomprises at least about 95% preprimitive streak cells.

155. The method of paragraph 153, wherein said enriched cell populationcomprises at least about 98% preprimitive streak cells.

156. The method of paragraph 153, wherein the differentiating stepcomprises providing said pluripotent cell population with at least onegrowth factor from the TGFβ superfamily in an amount sufficient topromote differentiation of said pluripotent cells to preprimitive streakcells.

157. The method of paragraph 156, wherein said at least one growthfactor of the TGFβ superfamily is activin A.

158. The method of paragraph 157, wherein said activin A is provided ina concentration of at least about 50 ng/ml.

159. The method of paragraph 157, wherein said activin A is provided ina concentration of at least about 100 ng/ml.

160. The method of paragraph 157, wherein said activin A is provided ina concentration of at least about 500 ng/ml.

161. The method of paragraph 153, wherein said cell population isdifferentiated in a medium comprising less than about 1% (v/v) serum.

162. The method of paragraph 153, wherein said cell population isdifferentiated in a medium comprising less than about 0.5% (v/v) serum.

163. The method of paragraph 153, wherein said cell population isdifferentiated in a medium comprising less than about 0.2% (v/v) serum.

164. The method of paragraph 153, wherein said cell population isdifferentiated in a medium comprising less than about 0.1% (v/v) serum.

165. The method of paragraph 153, wherein said cell population isdifferentiated in the absence of serum or the absence of serumreplacement.

166. The method of paragraph 153, wherein said cell population isdifferentiated in a medium comprising about 0% (v/v) serum on about thefirst day after adding said at least one growth factor, about 0.2% (v/v)serum or less on about the second day after adding said at least onegrowth factor and about 2% (v/v) serum or less on about the third dayafter adding said at least one growth factor.

167. The method of paragraph 153, wherein said cell population isdifferentiated in low serum RPMI medium.

168. An enriched population of preprimitive streak cells produced by themethod of paragraph 153.

169. A method of producing a cell population enriched in mesendodermcells, said method comprising the steps of obtaining a population ofpluripotent cells, wherein at least one cell of said pluripotent cellpopulation comprises at least one copy of a nucleic acid under thecontrol of a promoter selected from the group consisting of thebrachyury promoter, the FGF4 promoter and the SNAI1 promoter, saidnucleic acid comprising a sequence encoding green fluorescent protein(GFP) or a biologically active fragment thereof; differentiating saidpluripotent cells so as to produce mesendoderm cells, said mesendodermcells being multipotent cells that can differentiate into mesoderm cellsor definitive endoderm cells, and separating said mesendoderm cells fromcells which do not express GFP.

170. The method of paragraph 169, wherein said enriched cell populationcomprises at least about 95% mesendoderm cells.

171. The method of paragraph 169, wherein said enriched cell populationcomprises at least about 98% mesendoderm cells.

172. The method of paragraph 169, wherein the differentiating stepcomprises providing said pluripotent cell population with at least onegrowth factor of the TGFβ superfamily in an amount sufficient to promotedifferentiation of said pluripotent cells to mesendoderm cells.

173. The method of paragraph 172, wherein said at least one growthfactor of the TGFβ superfamily is activin A.

174. The method of paragraph 173, wherein said activin A is provided ina concentration of at least about 50 ng/ml.

175. The method of paragraph 173, wherein said activin A is provided ina concentration of at least about 100 ng/ml.

176. The method of paragraph 173, wherein said activin A is provided ina concentration of at least about 500 ng/ml.

177. The method of paragraph 169, wherein said cell population isdifferentiated in a medium comprising less than about 1% (v/v) serum.

178. The method of paragraph 169, wherein said cell population isdifferentiated in a medium comprising less than about 0.5% (v/v) serum.

179. The method of paragraph 169, wherein said cell population isdifferentiated in a medium comprising less than about 0.2% (v/v) serum.

180. The method of paragraph 169, wherein said cell population isdifferentiated in a medium comprising less than about 0.1% (v/v) serum.

181. The method of paragraph 169, wherein said cell population isdifferentiated in the absence of serum or the absence of serumreplacement.

182. The method of paragraph 169, wherein said cell population isdifferentiated in a medium comprising about 0% (v/v) serum on about thefirst day after adding said at least one growth factor, about 0.2% (v/v)serum or less on about the second day after adding said at least onegrowth factor and about 2% (v/v) serum or less on about the third dayafter adding said at least one growth factor.

183. The method of paragraph 169, wherein said cell population isdifferentiated in low serum RPMI medium.

184. An enriched population of mesendoderm cells produced by the methodof paragraph 169.

185. A method of identifying a differentiation factor capable ofpromoting the differentiation of preprimitive streak cells in a cellpopulation comprising human cells, said method comprising the steps ofobtaining a cell population comprising human preprimitive streak cells,providing a candidate differentiation factor to said cell population,determining expression of a marker in said cell population at a firsttime point, determining expression of the same marker in said cellpopulation at a second time point, wherein said second time point issubsequent to said first time point and wherein said second time pointis subsequent to providing said population with said candidatedifferentiation factor, and determining if expression of the marker insaid cell population at said second time point is increased or decreasedas compared to the expression of the marker in said cell population atsaid first time point, wherein an increase or decrease in expression ofsaid marker in said cell population indicates that said candidatedifferentiation factor is capable of promoting the differentiation ofsaid preprimitive streak cells.

186. The method of paragraph 185, wherein said human preprimitive streakcells comprise at least about 10% of the human cells in said cellpopulation.

187. The method of paragraph 185, wherein human feeder cells are presentin said cell population and wherein at least about 10% of the humancells other than said feeder cells are preprimitive streak cells.

188. The method of paragraph 185, wherein said human preprimitive streakcells comprise at least about 50% of the human cells in said cellpopulation.

189. The method of paragraph 185, wherein said human feeder cells arepresent in said cell population and wherein at least about 50% of thehuman cells other than said feeder cells are preprimitive streak cells.

190. The method of paragraph 185, wherein said human preprimitive streakcells differentiate into cells selected from the group consisting ofmesendoderm, mesoderm and definitive endoderm in response to saidcandidate differentiation factor.

191. The method of paragraph 185, wherein said human preprimitive streakcells differentiate into mesendoderm cells in response to said candidatedifferentiation factor.

192. The method of paragraph 191, wherein said marker is selected fromthe group consisting of brachyury, FGF4 and SNAI1.

193. The method of paragraph 185, wherein said human preprimitive streakcells differentiate into mesoderm cells in response to said candidatedifferentiation factor.

194. The method of paragraph 193, wherein said marker is selected fromthe group consisting of FOXF1, FLK1, BMP4, MOX1 and SDF1.

195. The method of paragraph 185, wherein said human preprimitive streakcells differentiate into definitive endoderm cells in response to saidcandidate differentiation factor.

196. The method of paragraph 195, wherein said marker is selected fromthe group consisting of CXCR4 and SOX17.

197. The method of paragraph 185, wherein said first time point is priorto providing said candidate differentiation factor to said cellpopulation.

198. The method of paragraph 185, wherein said first time point is atapproximately the same time as providing said candidate differentiationfactor to said cell population.

199. The method of paragraph 185, wherein said first time point issubsequent to providing said candidate differentiation factor to saidcell population.

200. The method of paragraph 185, wherein expression of said marker isincreased.

201. The method of paragraph 185, wherein expression of said marker isdecreased.

202. The method of paragraph 185, wherein expression of said marker isdetermined by quantitative polymerase chain reaction (Q-PCR).

203. The method of paragraph 185, wherein expression of said marker isdetermined by immunocytochemistry.

204. The method of paragraph 185, wherein said candidate differentiationfactor comprises at least one growth factor of the TGFβ superfamily.

205. The method of paragraph 204, wherein said at least one growthfactor of the TGFβ superfamily is activin A.

206. The method of paragraph 185, wherein said candidate differentiationfactor comprises a small molecule.

207. The method of paragraph 185, wherein said candidate differentiationfactor comprises a polypeptide.

208. The method of paragraph 185, wherein said candidate differentiationfactor is not a growth factor of the TGFβ superfamily.

209. The method of paragraph 185, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 0.1 ng/ml to about 10 mg/ml.

210. The method of paragraph 185, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 1 ng/ml to about 1 mg/ml.

211. The method of paragraph 185, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 10 ng/ml to about 100 μg/ml.

212. The method of paragraph 185, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 100 ng/ml to about 10 μg/ml.

213. The method of paragraph 185, wherein said candidate differentiationfactor is provided to said cell population at a concentration of about 1μg/ml.

214. A method of identifying a differentiation factor capable ofpromoting the differentiation of mesendoderm cells in a cell populationcomprising human cells, said method comprising the steps of obtaining acell population comprising human mesendoderm cells, providing acandidate differentiation factor to said cell population, determiningexpression of a marker in said cell population at a first time point,determining expression of the same marker in said cell population at asecond time point, wherein said second time point is subsequent to saidfirst time point and wherein said second time point is subsequent toproviding said population with said candidate differentiation factor,and determining if expression of the marker in said cell population atsaid second time point is increased or decreased as compared to theexpression of the marker in said cell population at said first timepoint, wherein an increase or decrease in expression of said marker insaid cell population indicates that said candidate differentiationfactor is capable of promoting the differentiation of said mesendodermcells.

215. The method of paragraph 214, wherein said human mesendoderm cellscomprise at least about 10% of the human cells in said cell population.

216. The method of paragraph 214, wherein human feeder cells are presentin said cell population and wherein at least about 10% of the humancells other than said feeder cells are mesendoderm cells.

217. The method of paragraph 214, wherein said human mesendoderm cellscomprise at least about 50% of the human cells in said cell population.

218. The method of paragraph 214, wherein said human feeder cells arepresent in said cell population and wherein at least about 50% of thehuman cells other than said feeder cells are mesendoderm cells.

219. The method of paragraph 214, wherein said human mesendoderm cellsdifferentiate into cells selected from the group consisting of mesodermand definitive endoderm in response to said candidate differentiationfactor.

220. The method of paragraph 214, wherein said human mesendoderm cellsdifferentiate into mesoderm cells in response to said candidatedifferentiation factor.

221. The method of paragraph 220, wherein said marker is selected fromthe group consisting of FOXF1, FLK1, BMP4, MOX1 and SDF1.

222. The method of paragraph 214, wherein said human mesendoderm cellsdifferentiate into definitive endoderm cells in response to saidcandidate differentiation factor.

223. The method of paragraph 222, wherein said marker is selected fromthe group consisting of CXCR4 and SOX17.

224. The method of paragraph 214, wherein said first time point is priorto providing said candidate differentiation factor to said cellpopulation.

225. The method of paragraph 214, wherein said first time point is atapproximately the same time as providing said candidate differentiationfactor to said cell population.

226. The method of paragraph 214, wherein said first time point issubsequent to providing said candidate differentiation factor to saidcell population.

227. The method of paragraph 214, wherein expression of said marker isincreased.

228. The method of paragraph 214, wherein expression of said marker isdecreased.

229. The method of paragraph 214, wherein expression of said marker isdetermined by quantitative polymerase chain reaction (Q-PCR).

230. The method of paragraph 214, wherein expression of said marker isdetermined by immunocytochemistry.

231. The method of paragraph 214, wherein said candidate differentiationfactor comprises at least one growth factor of the TGFβ superfamily.

232. The method of paragraph 231, wherein said at least one growthfactor of the TGFβ superfamily is activin A.

233. The method of paragraph 214, wherein said candidate differentiationfactor comprises a small molecule.

234. The method of paragraph 214, wherein said candidate differentiationfactor comprises a polypeptide.

235. The method of paragraph 214, wherein said candidate differentiationfactor is not a growth factor of the TGFβ superfamily.

236. The method of paragraph 214, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 0.1 ng/ml to about 10 mg/ml.

237. The method of paragraph 214, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 1 ng/ml to about 1 mg/ml.

238. The method of paragraph 214, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 10 ng/ml to about 100 μg/ml.

239. The method of paragraph 214, wherein said candidate differentiationfactor is provided to said cell population at a concentration of betweenabout 100 ng/ml to about 10 μg/ml.

240. The method of paragraph 214, wherein said candidate differentiationfactor is provided to said cell population at a concentration of about 1μg/ml.

241. A method of increasing the expression of the FGF8 gene product in ahuman embryonic stem cell (hESC) in vitro, said method comprisingobtaining said hESC in a medium comprising less than about 2% (v/v)serum and contacting said hESC with a differentiation factor in anamount sufficient to increase expression of the FGF8 gene product.

242. The method of paragraph 241, wherein said differentiation factorcomprises at least one growth factor of the TGFβ superfamily.

243. The method of paragraph 242, wherein said differentiation factorcomprises activin A.

244. The method of paragraph 241, wherein said medium lacks serum orlacks serum replacement.

245. A method of increasing the expression of a gene product selectedfrom the group consisting of brachyury, FGF4 and SNAI1 in an humanembryonic stem cell (hESC) or a preprimitive streak cell in vitro, saidmethod comprising obtaining said hESC or preprimitive streak cell in amedium comprising less than about 2% (v/v) serum and contacting saidhESC or said preprimitive streak cell with a differentiation factor inan amount sufficient to increase expression of a gene product selectedfrom the group consisting of brachyury, FGF4 and SNAI1.

246. The method of paragraph 245, wherein said differentiation factorcomprises at least one growth factor of the TGFβ superfamily.

247. The method of paragraph 246, wherein said differentiation factorcomprises activin A.

248. The method of paragraph 245, wherein said medium lacks serum orlacks serum replacement.

249. A cell culture comprising human embryonic stem cells (hESCs) and amedium comprising less than about 2% (v/v) serum, wherein said hESCsbegin differentiating at a reference time point such that expression ofFGF8 mRNA is substantially upregulated as compared to baseline FGF8 mRNAexpression in said hESCs by about 6 hours from said reference timepoint.

250. The cell culture of paragraph 249, wherein expression of FGF8 mRNAis downregulated after about 24 hours from said reference time point.

251. The cell culture of paragraph 250, wherein peak expression of FGF8mRNA is reached at a time between about 6 hours and about 24 hours fromsaid reference time point.

252. The cell culture of paragraph 249, wherein β-catenin polypeptidebegins to become localized to the cell nucleus by about 17 hours fromsaid reference time point.

253. The cell culture of paragraph 249, wherein expression of brachyurymRNA is substantially upregulated by about 24 hours from said referencetime point.

254. The cell culture of paragraph 253, wherein expression of brachyurymRNA is substantially downregulated by about 48 hours from saidreference time point.

255. The cell culture of paragraph 254, wherein peak expression ofbrachyury mRNA is reached at a time between about 12 hours and about 48hours from said reference time point.

256. The cell culture of paragraph 255, wherein brachyury mRNA in notsubstantially expressed by about 72 hours from said reference timepoint.

257. The cell culture of paragraph 249, wherein expression of FGF4 mRNAis substantially upregulated by about 24 hours from said reference timepoint.

258. The cell culture of paragraph 257, wherein expression of FGF4 mRNAis substantially downregulated by about 48 hours from said referencetime point.

259. The cell culture of paragraph 258, wherein peak expression of FGF4mRNA is reached at a time between about 12 hours and about 48 hours fromsaid reference time point.

260. The cell culture of paragraph 259, wherein FGF4 mRNA is notsubstantially expressed by about 72 hours from said reference timepoint.

261. The cell culture of paragraph 249, wherein expression of brachyuryand FGF4 mRNA is substantially upregulated by about 24 hours from saidreference time point.

262. The cell culture of paragraph 261, wherein expression of brachyuryand FGF4 mRNA is substantially downregulated by about 48 hours from saidreference time point.

263. The cell culture of paragraph 262, wherein peak expression ofbrachyury and FGF4 mRNA is reached at a time between about 12 hours andabout 48 hours from said reference time point.

264. The cell culture of paragraph 263, wherein brachyury and FGF4 mRNAin not substantially expressed by about 72 hours from said referencetime point.

265. The cell culture of paragraph 249, wherein expression of SNAI1 mRNAis substantially upregulated by about 24 hours from said reference timepoint.

266. The cell culture of paragraph 265, wherein expression of SNAI1 mRNAis downregulated by about 48 hours from said reference time point.

267. The cell culture of paragraph 266, wherein peak expression of SNAI1mRNA is reached at a time between about 12 hours and about 48 hours fromsaid reference time point.

268. The cell culture of paragraph 249, wherein expression of E-cadherinmRNA begins to be downregulated by about 12 hours from said referencetime point.

269. The cell culture of paragraph 249, wherein expression of E-cadherinmRNA is substantially downregulated by about 48 hours from saidreference time point.

270. The cell culture of paragraph 249, wherein expression of SOX17 mRNAis substantially upregulated by about 48 hours from said reference timepoint.

271. The cell culture of paragraph 249, wherein expression of FOXA2 mRNAis substantially upregulated by about 96 hours from said reference timepoint.

272. The cell culture of paragraph 249, wherein said medium comprisesless than about 1% (v/v) serum.

273. The cell culture of paragraph 249, wherein said medium comprisesless than about 0.2% (v/v) serum.

274. The cell culture of paragraph 249, wherein said medium comprisesabout 0% (v/v) serum.

275. The cell culture of paragraph 249, wherein said medium lacks serumor lacks serum replacement.

276. The cell culture of paragraph 249, further comprising adifferentiation factor of the TGFβ superfamily.

277. The cell culture of paragraph 276, wherein said differentiationfactor comprises activin A.

278. The cell culture of paragraph 277, wherein said activin A ispresent at a concentration of about 100 ng/ml.

279. A cell culture comprising human embryonic stem cells (hESCs), adifferentiation factor of the TGFβ superfamily and a medium comprisingless than about 2% (v/v) serum.

280. The cell culture of paragraph 279, wherein in cells of said cellculture, expression of a marker gene selected from the group consistingof FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

281. The cell culture of paragraph 280, wherein in cells of said cellculture, expression of a marker gene selected from the group consistingof FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior topeak expression of a marker gene selected from the group consisting ofbrachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662,SLIT2, GAD1 and GRM4.

282. The cell culture of paragraph 281, wherein in cells of said cellculture, expression of a marker gene selected from the group consistingof HEY1, GATA2, BIK and ID1 is downregulated prior to or at about thesame time as peak expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

283. The cell culture of paragraph 279, wherein in cells of said cellculture, expression of a marker gene selected from the group consistingof FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of SOX17, FOXA2, CXCR4 and MIXL1.

284. The cell culture of paragraph 283, wherein in cells of said cellculture, expression of a marker gene selected from the group consistingof brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662,SLIT2, GAD1 and GRM4 is upregulated prior to or at about the same timeas upregulation of expression of a marker gene selected from the groupconsisting of SOX17, FOXA2, CXCR4 and MIXL1.

285. The cell culture of paragraph 284, wherein in cells of said cellculture, peak expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4 is reached prior to or at aboutthe same time as upregulation of expression of a marker gene selectedfrom the group consisting of SOX17, FOXA2, CXCR4 and MIXL1.

286. The cell culture of paragraph 283, wherein in cells of said cellculture, expression of a marker gene selected from the group consistingof FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

287. The cell culture of paragraph 279, wherein said medium comprisesless than about 1% (v/v) serum.

288. The cell culture of paragraph 279, wherein said medium comprisesless than about 0.2% (v/v) serum.

289. The cell culture of paragraph 279, wherein said medium comprisesabout 0% (v/v) serum.

290. The cell culture of paragraph 279, wherein said medium lacks serumor lacks serum replacement.

291. The cell culture of paragraph 279, wherein said differentiationfactor of the TGFβ superfamily comprises activin A.

292. The cell culture of paragraph 291, wherein said activin A ispresent in the medium at a concentration of about 100 ng/ml.

293. A method of differentiating cells in a cell culture, said methodcomprising (a) contacting a cell culture comprising human embryonic stemcells (hESCs) with a medium comprising less that about 2% serum, (b)providing said hESCs with a differentiation factor of the TGFβsuperfamily, and (c) permitting differentiation of said hESCs to occur.

294. The method of paragraph 283, wherein in cells of said cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

295. The method of paragraph 294, wherein in cells of said cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior to peakexpression of a marker gene selected from the group consisting ofbrachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662,SLIT2, GAD1 and GRM4.

296. The method of paragraph 295, wherein in cells of said cell culture,expression of a marker gene selected from the group consisting of HEY1,GATA2, BIK and ID1 is downregulated prior to or at about the same timeas peak expression of a marker gene selected from the group consistingof brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662,SLIT2, GAD1 and GRM4.

297. The method of paragraph 293, wherein in cells of said cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of SOX17, FOXA2, CXCR4 and MIXL1.

298. The method of paragraph 297, wherein in cells of said cell culture,expression of a marker gene selected from the group consisting ofbrachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662,SLIT2, GAD1 and GRM4 is upregulated prior to or at about the same timeas upregulation of expression of a marker gene selected from the groupconsisting of SOX17, FOXA2, CXCR4 and MIXL1.

299. The method of paragraph 298, wherein in cells of said cell culture,peak expression of a marker gene selected from the group consisting ofbrachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662,SLIT2, GAD1 and GRM4 is reached prior to or at about the same time asupregulation of expression of a marker gene selected from the groupconsisting of SOX17, FOXA2, CXCR4 and MIXL1.

300. The method of paragraph 297, wherein in cells of said cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

301. The method of paragraph 293, wherein said medium comprises lessthan about 1% (v/v) serum.

302. The method of paragraph 293, wherein said medium comprises lessthan about 0.2% (v/v) serum.

303. The method of paragraph 293, wherein said medium comprises about 0%(v/v) serum.

304. The method of paragraph 293, wherein said medium lacks serum orlacks serum replacement.

305. The method of paragraph 293, wherein said differentiation factor ofthe TGFβ superfamily comprises activin A.

306. The method of paragraph 305, wherein said activin A is present inthe medium at a concentration of about 100 ng/ml.

307. A cell culture comprising human embryonic stem cells (hESCs) and amedium comprising less than about 2% (v/v) serum, wherein said hESCsbegin differentiating at a reference time point such that expression ofan mRNA selected from the group consisting of FZD10, FGF5 and OCT4 issubstantially unregulated as compared to baseline expression of acorresponding mRNA selected from the group consisting of FZD10, FGF5 andOCT4 in said hESCs by about 2 hours from said reference time point.

308. The cell culture of paragraph 307, wherein said medium comprisesless than about 1% (v/v) serum.

309. The cell culture of paragraph 307, wherein said medium comprisesless than about 0.2% (v/v) serum.

310. The cell culture of paragraph 307, wherein said medium comprisesabout 0% (v/v) serum.

311. The cell culture of paragraph 307, wherein said medium lacks serumor lacks serum replacement.

312. The cell culture of paragraph 307 further comprising adifferentiation factor of the TGFβ superfamily.

313. The cell culture of paragraph 312, wherein said differentiationfactor comprises activin A.

314. The cell culture of paragraph 313, wherein said activin A ispresent at a concentration of about 100 ng/ml.

315. A cell culture comprising human embryonic stem cells (hESCs) and amedium comprising less than about 2% (v/v) serum, wherein said hESCsbegin differentiating at a reference time point such that expression ofan mRNA selected from the group consisting of GBX2, ZFP42 and SOX2 issubstantially downregulated as compared to baseline expression of acorresponding mRNA selected from the group consisting of GBX2, ZFP42 andSOX2 in said hESCs by about 2 hours from said reference time point.

316. The cell culture of paragraph 315, wherein said medium comprisesless than about 1% (v/v) serum.

317. The cell culture of paragraph 315, wherein said medium comprisesless than about 0.2% (v/v) serum.

318. The cell culture of paragraph 315, wherein said medium comprisesabout 0% (v/v) serum.

319. The cell culture of paragraph 315, wherein said medium lacks serumor lacks serum replacement.

320. The cell culture of paragraph 315 further comprising adifferentiation factor of the TGFβ superfamily.

321. The cell culture of paragraph 320, wherein said differentiationfactor comprises activin A.

322. The cell culture of paragraph 321, wherein said activin A ispresent at a concentration of about 100 ng/ml.

323. A cell culture comprising human embryonic stem cells (hESCs), adifferentiation factor of the TGFβ superfamily and a medium comprisingless than about 2% (v/v) serum, wherein in cells of said cell culture,expression of a marker gene selected from the group consisting of FZD10,FGF5, Nanog and OCT4 is upregulated or expression of a marker geneselected from the group consisting of GBX2, ZFP42 and SOX2 isdownregulated prior to upregulation of expression of a marker geneselected from the group consisting of FGF8, Nodal, HEG, HEY1, GATA2, BIKand ID1.

324. The cell culture of paragraph 323, wherein expression of a markergene selected from the group consisting of FZD10, FGF5, Nanog and OCT4is upregulated or expression of a marker gene selected from the groupconsisting of GBX2, ZFP42 and SOX2 is downregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

325. The cell culture of paragraph 323, wherein said medium comprisesless than about 1% (v/v) serum.

326. The cell culture of paragraph 323, wherein said medium comprisesless than about 0.2% (v/v) serum.

327. The cell culture of paragraph 323, wherein said medium comprisesabout 0% (v/v) serum.

328. The cell culture of paragraph 323, wherein said medium lacks serumor lacks serum replacement.

329. The cell culture of paragraph 323 further comprising adifferentiation factor of the TGFβ superfamily.

330. The cell culture of paragraph 329, wherein said differentiationfactor comprises activin A.

331. The cell culture of paragraph 330, wherein said activin A ispresent at a concentration of about 100 ng/ml.

332. A method of differentiating cells in a cell culture, said methodcomprising (a) contacting a cell culture comprising human embryonic stemcells (hESCs) with a medium comprising less that about 2% serum, (b)providing said hESCs with a differentiation factor of the TGFβsuperfamily, and (c) permitting differentiation of said hESCs to occur,wherein in cells of said cell culture, expression of a marker geneselected from the group consisting of FZD10, FGF5, Nanog and OCT4 isupregulated or expression of a marker gene selected from the groupconsisting of GBX2, ZFP42 and SOX2 is downregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1.

333. The method of paragraph 332, wherein expression of a marker geneselected from the group consisting of FZD10, FGF5, Nanog and OCT4 isupregulated or expression of a marker gene selected from the groupconsisting of GBX2, ZFP42 and SOX2 is downregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

334. The method of paragraph 332, wherein said medium comprises lessthan about 1% (v/v) serum.

335. The method of paragraph 332, wherein said medium comprises lessthan about 0.2% (v/v) serum.

336. The method of paragraph 332, wherein said medium comprises about 0%(v/v) serum.

337. The method of paragraph 332, wherein said medium lacks serum orlacks serum replacement.

338. The method of paragraph 332 further comprising a differentiationfactor of the TGFβ superfamily.

339. The method of paragraph 338, wherein said differentiation factorcomprises activin A.

340. The method of paragraph 339, wherein said activin A is present at aconcentration of about 100 ng/ml.

It will be appreciated that the methods and compositions described aboverelate to cells cultured in vitro. However, the above-described in vitrodifferentiated cell compositions may be used for in vivo applications.

Additional embodiments of the present invention may also be found inU.S. Provisional Patent Application No. 60/532,004, entitled DEFINITIVEENDODERM, filed Dec. 23, 2003; U.S. Provisional Patent Application No.60/566,293, entitled PDX1 EXPRESSING ENDODERM, filed Apr. 27, 2004; U.S.Provisional Patent Application No. 60/586,566, entitled CHEMOKINE CELLSURFACE RECEPTOR FOR THE ISOLATION OF DEFINITIVE ENDODERM, filed Jul. 9,2004; U.S. Provisional Patent Application No. 60/587,942, entitledCHEMOKINE CELL SURFACE RECEPTOR FOR THE ISOLATION OF DEFINITIVEENDODERM, filed Jul. 14, 2004; U.S. patent application Ser. No.11/021,618, entitled DEFINITIVE ENDODERM, filed Dec. 23, 2004, U.S.patent application Ser. No. 11/115,868, entitled PDX1 EXPRESSINGENDODERM, filed Apr. 26, 2005, U.S. patent application Ser. No.11/165,305, entitled METHODS FOR IDENTIFYING FACTORS FOR DIFFERENTIATINGDEFINITIVE ENDODERM, filed Jun. 23, 2005 and U.S. Provisional PatentApplication No. 60/693,364, entitled PREPRIMITIVE STREAK CELLS ANDMESENDODERM CELLS, filed Jun. 23, 2005, the disclosures of which areincorporated herein by reference in their entireties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting the differentiation of embryonic stemcells (ESC) in the presence of activins, in the presence of acombination of BMP4 and SU5402, and in the absence of anydifferentiation factors. Some useful marker genes for the identificationand/or detection of each cell type are also listed.

FIGS. 2A-M are bar charts showing the expression patterns of markergenes that can be used to identify definitive endoderm cells. Theexpression analysis of definitive endoderm markers, FGF17, VWF, CALCR,FOXQ1, CMKOR1 and CRIP1 is shown in panels G-L, respectively. Theexpression analysis of previously described lineage marking genes,SOX17, SOX7, SOX17/SOX7, TM, ZIC1, and MOX1 is shown in panels A-F,respectively. Panel M shows the expression analysis of CXCR4. Withrespect to each of panels A-M, the column labeled hESC indicates geneexpression from purified human embryonic stem cells; 2NF indicates cellstreated with 2% FBS, no activin addition; 0.1A100 indicates cellstreated with 0.1% FBS, 100 ng/ml activin A; 1A100 indicates cellstreated with 1% FBS, 100 ng/ml activin A; and 2A100 indicates cellstreated with 2% FBS, 100 ng/ml activin A.

FIGS. 3A-L are bar charts showing the expression patterns of certainmarker genes expressed in (A) preprimitive streak cells, (B-D) primitivestreak (mesendoderm) cells, (E-F) mesendoderm and definitive endodermcells, (G) definitive endoderm cells, and (H-L) trophectoderm andprimitive endoderm cells. Marker expression was determined by Q-PCRduring the course of differentiation at 0, 6, 12, 24, 48, 72 and 96hours after addition of 100 ng/ml activin A (indicated by “A”) or acombination of 100 ng/ml of BMP4 and 2.5 μM SU5402 (indicated as “B/SU”)to the cell cultures. All cells were grown in RPMI medium containing 0%(v/v) FBS on day 1 (first 24 hours), 0.2% (v/v) FBS on day 2 (24-48hours) and 2% (v/v) FBS on days 3 and 4 (48-96 hours). The full name ofeach marker gene is provided in Table 1.

FIGS. 4A-C are bar charts showing the mRNA expression patterns of (A)the Brachyury marker for mesendoderm (BRACH), (B) the E-cadherin markerfor hESCs, and (C) the SNAI1 marker for mesendoderm during the first 48hours of differentiation from hESCs (indicated by “ESC”) in the presenceof 100 ng/ml activin A and 0.1% (v/v) FBS.

FIGS. 5A and B are bar charts showing the mRNA expression patterns ofthe Brachyury marker for mesendoderm (BRACH) and the Sex DeterminingRegion Y-Box 17 (SOX17) marker for definitive endoderm, respectively,during the first 48 hours of differentiation from hESCs (indicated by“ES”) in the presence of 100 ng/ml activin A and 0.1% (v/v) FBS.

FIGS. 6A-D are micrographs which show cells 48 hours afterdifferentiation from hESCs in the presence of 100 ng/ml activin A and0.1% (v/v) FBS. Panel A shows a field of cells stained with DAPI. PanelsB-D show the same field of cells stained using a primary antibodiesagainst Brachyury (B), SOX17 (C), or both Brachyury and SOX17 (D).

FIG. 7 is a diagram depicting the differentiation of embryonic stemcells (ESC) under low serum conditions in the presence of 100 ng/mlactivin A (A100) or in its absence (NF). PS indicates primitive streakcells (mesendoderm cells); DE indicates definitive endoderm cells; and Mindicates mesoderm cells.

FIGS. 8A-F are bar charts showing the expression of the mesoderm markergenes, (A) BRACH, (B) FOXF1, (C) FLK1, (D) BMP4, (E) MOX1 and (F) SDF1,in cell cultures after four days of incubation in the continued presenceof 100 ng/ml activin A (A100) or after four days of incubation whereinactivin A is withdrawn after the first day (NF). Marker expression wasdetermined by Q-PCR.

FIGS. 9A-C are bar charts showing the expression of the definitiveendoderm marker genes, (A) GSC, (B) SOX17 and (C) FOXA2, in cellcultures after four days of incubation in the continued presence of 100ng/ml activin A (A100) or after four days of incubation wherein activinA is withdrawn after the first day (NF). Marker expression wasdetermined by Q-PCR.

FIGS. 10A-I are bar charts showing the expression patterns of certainmarker genes useful for the identification and/or detection ofdefinitive endoderm cells. Marker expression was determined by Q-PCRfrom five-day-old cell cultures containing fetal bovine serum (FBS) at aconcentration of 0.5%, 2% or 10% and either 100 ng/ml activin A (A100)or no activin A (NF).

FIGS. 11A and B are bar charts showing the mRNA expression patterns ofdefinitive endoderm markers (A) SOX17 and (B) FOXA2 in cell culturesdifferentiated in the presence of 100 ng/ml activin A for four days.Marker expression was determined by Q-PCR during the course ofdifferentiation at 0, 6, 24, 30, 48 and 96 hours after addition ofactivin A.

FIGS. 12A-D show the in vivo differentiation of definitive endodermcells that are transplanted under the kidney capsule ofimmunocompromised mice. Panels: A—hetatoxylin-eosin staining showinggut-tube-like structures; B—antibody immunoreactivity against hepatocytespecific antigen (liver); C—antibody immunoreactivity against villin(intestine); and D—antibody immunoreactivity against CDX2 (intestine).

DETAILED DESCRIPTION

A crucial stage in early human development termed gastrulation occurs2-3 weeks after fertilization. Gastrulation is extremely significantbecause it is at this time that the three primary germ layers are firstspecified and organized (Lu et al., 2001; Schoenwolf and Smith, 2000).The ectoderm is responsible for the eventual formation of the outercoverings of the body and the entire nervous system whereas the heart,blood, bone, skeletal muscle and other connective tissues are derivedfrom the mesoderm. Definitive endoderm is defined as the germ layer thatis responsible for formation of the entire gut tube which includes theesophagus, stomach and small and large intestines, and the organs whichderive from the gut tube such as the lungs, liver, thymus, parathyroidand thyroid glands, gall bladder and pancreas (Grapin-Botton and Melton,2000; Kimelman and Griffin, 2000; Tremblay et al., 2000; Wells andMelton, 1999; Wells and Melton, 2000). A very important distinctionshould be made between the definitive endoderm and the completelyseparate lineage of cells termed primitive endoderm. The primitiveendoderm is primarily responsible for formation of extra-embryonictissues, mainly the parietal and visceral endoderm portions of theplacental yolk sac and the extracellular matrix material of Reichert'smembrane.

During gastrulation, the process of definitive endoderm formation beginswith a cellular migration event in which mesendoderm cells (cellscompetent to form mesoderm or endoderm) migrate through a structurecalled the primitive streak. As the mesendoderm cells ingress throughthe primitive streak, they undergo an epithelial to mesenchymaltransition (EMT) and become either mesoderm or definitive endoderm.Definitive endoderm is derived from cells, which migrate through theanterior portion of the streak and through the node (a specializedstructure at the anterior-most region of the streak). As migrationoccurs, definitive endoderm populates first the most anterior gut tubeand culminates with the formation of the posterior end of the gut tube.

Both primitive streak precursor cells (preprimitive streak cells) andprimitive streak cells (mesendoderm cells) are early stage precursorcells the give rise to the mesoderm and the definitive endoderm. Infact, preprimitive streak cells and mesendoderm cells may be theearliest precursors in the developmental process from pluripotency toterminally differentiated cells, tissues and/or organs made from themesoderm and definitive endoderm lineages. Until now, neither cellpopulations enriched in human preprimitive streak cells nor cellpopulations enriched in human mesendoderm cells have been obtained.Furthermore, the cells of such cell populations have not been previouslycharacterized in vitro.

In view of the foregoing, some embodiments of the invention describedherein relate to cell cultures and/or enriched cell populationscomprising human preprimitive streak cells and cell cultures and/orenriched cell populations comprising human mesendoderm cells. In suchembodiments, the preprimitive streak cells and the mesendoderm cells arecapable of further differentiation into mesoderm cells and/or definitiveendoderm cells as well as cells, tissues and/or organs derived fromthese lineages.

Other embodiments of the invention relate to methods for producing cellcultures and/or enriched cell populations comprising human preprimitivestreak cells as well as methods for producing cell cultures and/orenriched cell populations comprising human mesendoderm cells.

Still other embodiments described herein relate to screening methods foridentifying one or more differentiation factors that are useful fordifferentiating cells in a cell population comprising preprimitivestreak cells or mesendoderm cells. Such factors are useful for promotingthe differentiation of these cell types to mesoderm and/or definitiveendoderm cells as well as cells, tissues and/or organs derived fromeither of these cell lineages.

Certain other aspects of the present invention relate to methods forincreasing the expression of certain early stage cell markers. Furtheraspects relate to cell compositions comprising cells expressing certainmarkers during the course of differentiation.

DEFINITIONS

Certain terms and phrases as used throughout this application have themeanings provided as follows:

As used herein, “embryonic” refers to a range of developmental stages ofan organism beginning with a single zygote and ending with amulticellular structure that no longer comprises pluripotent ortotipotent cells other than developed gametic cells. In addition toembryos derived by gamete fusion, the term “embryonic” refers to embryosderived by somatic cell nuclear transfer.

As used herein, “multipotent” or “multipotent cell” refers to a celltype that can give rise to a limited number of other particular celltypes.

As used herein, “expression” refers to the production of a material orsubstance as well as the level or amount of production of a material orsubstance. Thus, determining the expression of a specific marker refersto detecting either the relative or absolute amount of the marker thatis expressed or simply detecting the presence or absence of the marker.

As used herein, “marker” refers to any molecule that can be observed ordetected. For example, a marker can include, but is not limited to, anucleic acid, such as a transcript of a specific gene, a polypeptideproduct of a gene, a non-gene product polypeptide, a glycoprotein, acarbohydrate, a glycolipd, a lipid, a lipoprotein or a small molecule(for example, molecules having a molecular weight of less than 10,000amu)

When used in connection with cell cultures and/or cell populations, theterm “portion” means any non-zero amount of the cell culture or cellpopulation, which ranges from a single cell to the entirety of the cellculture or cells population.

With respect to cells in cell cultures or in cell populations, thephrase “substantially free of” means that the specified cell type ofwhich the cell culture or cell population is free, is present in anamount of less than about 5% of the total number of cells present in thecell culture or cell population.

With respect to cell culture medium, as used herein, “low serum RPMI”refers to a low serum containing medium, wherein the serum concentrationis gradually increased over a defined time period. For example, in oneembodiment, low serum RPMI comprises a concentration of about 0.2% fetalbovine serum (FBS) on the first day of cell growth, about 0.5% FBS onthe second day of cell growth and about 2% FBS on the third throughfifth day of cell growth. In another embodiment, low serum RPMIcomprises a concentration of about 0% on day one, about 0.2% on day twoand about 2% on the third and subsequent days.

As used herein, “serum replacement” refers to serum substitutecomprising IGF or insulin.

Model of Early Events in the Differentiation of hESCs to DefinitiveEndoderm Cells

FIG. 1 displays a model summarizing the early transitions of humanembryonic stem cells (hESCs) in vitro. Differentiation of hESCs througha process that closely recapitulates gastrulation can be orchestrated bythe application of high dose activin A in the context of low serumsupplementation. The expression of FGF8 and nuclear localization ofβ-catenin, events that occur in the proximal epiblast prior to primitivestreak formation, is evident prior to about 24 hours (preprimitivestreak cells). High level expression of the primitive streak-expressedgenes (brachyury and FGF4) occurs at about 24 hours. If maintained inhigh dose activin A, the primitive streak cells (mesendoderm cells) areefficiently converted into definitive endoderm. In contrast, in theabsence of activins, these mesendoderm precursors become mesoderm.Treatment of hESCs with BMP4 and SU5402 induces gene expressionassociated with primitive endoderm and trophectoderm.

Preprimitive Streak Cells and Mesendoderm Cells and Processes RelatedThereto

Embodiments described herein relate to novel, defined processes for theproduction of preprimitive streak cells and/or mesendoderm cells inculture by differentiating pluripotent cells, such as stem cells intopreprimitive streak cells and/or mesendoderm cells. As described above,preprimitive streak cells are capable of differentiating intomesendoderm cells as well as cells, tissues and/or organs derivedtherefrom. Mesendoderm cells are capable of differentiating intomesoderm cells and/or definitive endoderm cells as well as cells,tissues and/or organs derived from either of these lineages. In someembodiments, the preprimitive steak cells are converted, through amesendoderm intermediate, into terminally differentiated cells of eitherthe mesoderm or definitive endoderm lineages. As will be described infurther detail below, such processes can provide the basis for efficientproduction of a variety of human endodermal and mesodermal derivedtissues. For example, such processes can provide the basis for efficientproduction of human endodermal derived tissues, such as pancreas, liver,lungs, stomach, intestine, thyroid, thymus, pharynx, gallbladder andurinary bladder. Importantly, production of preprimitive streak cellsand/or mesendoderm cells is an early step in differentiation of a stemcell to a functional insulin-producing β-cell. As another example,preprimitive streak cell and/or mesendoderm cell differentiation canprovide the basis for efficient production of human mesodermal derivedtissues, such as blood cells, cardiovascular tissues, skeletal tissuesas well as other structural and connective tissues. To obtain usefulquantities of any of the above-described cell or tissue types, highefficiency differentiation is desirable for each of the differentiationsteps that occur prior to reaching the terminally differentiated cellfate. Since differentiation of stem cells to preprimitive streak cellsand/or mesendoderm cells represents very early steps towards theproduction of functional terminally differentiated cells of the mesodermand definitive endoderm cell lineages (as shown in FIG. 1), highefficiency differentiation at this step is particularly desirable.

In view of the desirability of efficient differentiation of pluripotentcells to preprimitive streak cells and/or mesendoderm cells, someaspects of the differentiation processes described herein relate to invitro methodology that results in approximately 5-90% conversion ofpluripotent cells to preprimitive streak cells and/or mesendoderm cells.Typically, such methods encompass the application of culture and growthfactor conditions in a defined and temporally specified fashion. Furtherenrichment of the cell population for preprimitive streak cells and/ormesendoderm cells can be achieved by isolation and/or purification ofthe preprimitive streak cells and/or mesendoderm cells from other cellsin the population by sorting cells based on differential fluorescentmarker expression. As such, some embodiments described herein relate topreprimitive streak cells as well as methods for producing and isolatingand/or purifying such cells. Other embodiments relate to mesendodermcells as well as methods for producing and isolating and/or purifyingsuch cells.

In order to determine the amount of preprimitive streak cells in a cellculture or cell population, a method of distinguishing this cell typefrom the other cells in the culture or in the population is desirable.Accordingly, certain embodiments described herein relate to cell markerswhose presence, absence and/or relative expression levels are specificfor preprimitive streak cells and methods for detecting and determiningthe expression of such markers.

In some embodiments described herein, the presence, absence and/or levelof expression of a marker is determined by quantitative PCR (Q-PCR). Forexample, the amount of transcript produced by certain genetic markers,such as OCT4, ECAD, FGF8, β-catenin, brachyury, FGF4, SNAI1, SOX17,CXCR4, GSC, MIXL1, FOXA2, SOX7, FOXF1, FLK1, BML4, MOX1, SDF1 and othermarkers described herein is determined by quantitative Q-PCR. In otherembodiments, immunohistochemistry is used to detect the proteinsexpressed by the above-mentioned genes. In still other embodiments,immunohistochemistry/immunocytochemistry is used to detect cellcompartmental localization of certain polypeptide markers, such as thenuclear localization of β-catenin. In yet other embodiments, Q-PCR andimmunohistochemical techniques are both used to identify and determinethe amount or relative proportions of such markers.

By using methods, such as those described above, to determine theexpression of one or more appropriate markers, it is possible toidentify preprimitive streak cells and/or mesendoderm cells, as well asdetermine the proportion of preprimitive streak cells and/or mesendodermcells in a cell culture or cell population. For example, in someembodiments of the present invention, the preprimitive streak cells orcell populations that are produced express the FGF8 marker and/ornuclear-localized β-catenin at a level of about 2 orders of magnitudegreater than non-preprimitive streak cell types or cell populations. Inother embodiments, the preprimitive streak cells or cell populationsthat are produced express the FGF8 marker and/or nuclear-localizedβ-catenin at a level of more than 2 orders of magnitude greater thannon-preprimitive streak cell types or cell populations. In otherembodiments of the present invention, the mesendoderm cells or cellpopulations that are produced express the brachyury, FGF4 and/or SNAI1markers at a level of about 2 orders of magnitude greater thannon-mesendoderm cell types or cell populations. In other embodiments,the mesendoderm cells or cell populations that are produced express thebrachyury, FGF4 and/or SNAI1 markers at a level of more than 2 orders ofmagnitude greater than non-mesendoderm cell types or cell populations.

Embodiments described herein also relate to preprimitive streak and/ormesendoderm compositions. For example, some embodiments relate to cellcultures comprising preprimitive streak cells and/or mesendoderm cells,whereas others relate to cell populations enriched in preprimitivestreak cells and/or mesendoderm cells. Some preferred embodiments relateto cell cultures which comprise preprimitive streak cells and/ormesendoderm cells, wherein at least about 5-90% of the cells in cultureare preprimitive streak cells and/or mesendoderm cells. An especiallypreferred embodiment relates to cells cultures comprising human cells,wherein at least about 5-90% of the human cells in culture arepreprimitive streak cells and/or mesendoderm cells. Because theefficiency of the differentiation procedure can be adjusted by modifyingcertain parameters, which include but are not limited to, cell growthconditions, growth factor concentrations and the timing of culturesteps, the differentiation procedures described herein can result inabout 5%, about 10%, about 15%, about 20%, about 25%, about 30%, about35%, about 40%, about 45%, about 50%, about 55%, about 60%, about 65%,about 70%, about 75%, about 80%, about 85%, about 90%, about 95%, orgreater than about 95% conversion of pluripotent cells to preprimitivestreak cells and/or mesendoderm cells. In other preferred embodiments,conversion of a pluripotent cell population, such as a stem cellpopulation, to substantially pure preprimitive streak cell and/ormesendoderm cell population is contemplated.

The compositions and methods described herein have several usefulfeatures. For example, the cell cultures and cell populations comprisingpreprimitive streak cells and/or mesendoderm cells as well as themethods for producing such cell cultures and cell populations are usefulfor modeling the early stages of human development. Furthermore, thecompositions and methods described herein can also serve for therapeuticintervention in disease states, such as diabetes mellitus. For example,since preprimitive streak cells and/or mesendoderm cells serve as thesource for only a limited number of tissues, they can be used in thedevelopment of pure tissue or cell types.

Production of Preprimitive Streak Cells from Pluripotent Cells.

In some processes for producing preprimitive streak cells, thepluripotent cells used as starting material are stem cells. In certainprocesses, preprimitive streak cell cultures and enriched cellpopulations comprising preprimitive streak cells are produced fromembryonic stem cells. A preferred method for deriving preprimitivestreak cells utilizes human embryonic stem cells as the startingmaterial for preprimitive streak cell production. Such pluripotent cellscan be cells that originate from the morula, embryonic inner cell massor those obtained from embryonic gonadal ridges. Human embryonic stemcells can be maintained in culture in a pluripotent state withoutsubstantial differentiation using methods that are known in the art.Such methods are described, for example, in U.S. Pat. Nos. 5,453,357,5,670,372, 5,690,926 5,843,780, 6,200,806 and 6,251,671 the disclosuresof which are incorporated herein by reference in their entireties.

In some processes for producing preprimitive streak cells, hESCs aremaintained on a feeder layer. In such processes, any feeder layer whichallows hESCs to be maintained in a pluripotent state can be used. Onecommonly used feeder layer for the cultivation of human embryonic stemcells is a layer of mouse fibroblasts. More recently, human fibroblastfeeder layers have been developed for use in the cultivation of hESCs(see US Patent Application No. 2002/0072117, the disclosure of which isincorporated herein by reference in its entirety). Alternative processesfor producing preprimitive streak cells permit the maintenance ofpluripotent hESC without the use of a feeder layer. Methods ofmaintaining pluripotent hESCs under feeder-free conditions have beendescribed in US Patent Application No. 2003/0175956, the disclosure ofwhich is incorporated herein by reference in its entirety.

The human embryonic stem cells used herein can be maintained in cultureeither with or without serum. In some embryonic stem cell maintenanceprocedures, serum replacement is used. In others, serum free culturetechniques, such as those described in US Patent Application No.2003/0190748, the disclosure of which is incorporated herein byreference in its entirety, are used.

Stem cells are maintained in culture in a pluripotent state by routinepassage until it is desired that they be differentiated intopreprimitive streak cells. In some processes, differentiation topreprimitive streak cells is achieved by providing to the stem cellculture a differentiation factor, such as a growth factor of the TGFβsuperfamily, in an amount sufficient to promote differentiation topreprimitive streak cells. Growth factors of the TGFβ superfamily whichare useful for the production of preprimitive streak cells are selectedfrom the Nodal/Activin subgroups. In some preferred differentiationprocesses, the growth factor is selected from the group consisting ofNodal, activin A, and activin B. In certain differentiation processes,the growth factor activin A is used.

With respect to some of the processes for the differentiation ofpluripotent stem cells to preprimitive streak cells, the above-mentionedgrowth factors are provided to the cells so that the growth factors arepresent in the cultures at concentrations sufficient to promotedifferentiation of at least a portion of the stem cells to preprimitivestreak cells. In some processes, the above-mentioned growth factors arepresent in the cell culture at a concentration of at least about 5ng/ml, at least about 10 ng/ml, at least about 25 ng/ml, at least about50 ng/ml, at least about 75 ng/ml, at least about 100 ng/ml, at leastabout 200 ng/ml, at least about 300 ng/ml, at least about 400 ng/ml, atleast about 500 ng/ml, at least about 1000 ng/ml, at least about 2000ng/ml, at least about 3000 ng/ml, at least about 4000 ng/ml, at leastabout 5000 ng/ml or more than about 5000 ng/ml.

In certain processes for the differentiation of pluripotent stem cellsto preprimitive streak cells, the above-mentioned growth factors areremoved from the cell culture subsequent to their addition. For example,the growth factors can be removed within about 1 hour, within about 2hours, within about 3 hours, within about 4 hours, within about 5 hours,within about 6 hours, within about 7 hours, within about 8 hours, withinabout 9 hours, within about 10 hours, within about 11 hours, withinabout 12 hours, within about 13 hours, within about 14 hours, withinabout 15 hours, within about 16 hours, within about 17 hours, withinabout 18 hours, within about 19 hours, within about 20 hours, withinabout 21 hours, within about 22 hours, within about 23 hours, withinabout 24 hours or within about more than 24 hours.

Cultures of preprimitive streak cells can be grown in medium containingreduced serum or no serum. Under certain culture conditions, serumconcentrations can range from about 0% (v/v) to about 10% (v/v). Forexample, in some differentiation processes, the serum concentration ofthe medium can be less than about 0.05% (v/v), less than about 0.1%(v/v), less than about 0.2% (v/v), less than about 0.3% (v/v), less thanabout 0.4% (v/v), less than about 0.5% (v/v), less than about 0.6%(v/v), less than about 0.7% (v/v), less than about 0.8% (v/v), less thanabout 0.9% (v/v), less than about 1% (v/v), less than about 2% (v/v),less than about 3% (v/v), less than about 4% (v/v), less than about 5%(v/v), less than about 6% (v/v), less than about 7% (v/v), less thanabout 8% (v/v), less than about 9% (v/v) or less than about 10% (v/v).In some processes, preprimitive streak cells are grown without serum orwithout serum replacement. In still other processes, preprimitive streakcells are grown in the presence of B27. In such processes, theconcentration of B27 supplement can range from about 0.1% (v/v) to about20% (v/v).

Monitoring the Differentiation of Pluripotent Cells to PreprimitiveStreak Cells

The progression of the hESC culture to preprimitive streak cells can bemonitored by determining the temporal expression of markerscharacteristic of preprimitive streak cells. In some processes, theexpression of certain markers is determined by detecting the presence orabsence of the marker. Alternatively, the expression of certain markerscan be determined by measuring the level at which the marker is presentin the cells of the cell culture or cell population at one or more timepoints subsequent to the addition of the differentiation factor. In suchprocesses, the measurement of marker expression can be qualitative orquantitative. One method of quantitating the expression of markers thatare produced by marker genes is through the use of quantitativepolymerase chain reaction (Q-PCR). Methods of performing Q-PCR are wellknown in the art. Other methods which are known in the art can also beused to quantitate marker gene expression. For example, the expressionof a marker gene product can be detected by using antibodies specificfor the marker gene product of interest. In certain processes, theexpression of marker genes characteristic of preprimitive streak cellsas well as the lack of significant expression of marker genescharacteristic of hESCs and other cell types is determined. In stillother processes, both the timing and amount of expression of markergenes characteristic of preprimitive streak cells at one or more timepoints subsequent to the addition of the differentiation factor isdetermined.

As described further in the Examples below, markers of preprimitivestreak cells are FGF8 and β-catenin. As such, the preprimitive streakcells produced by the processes described herein express the FGF8 andβ-catenin marker genes, thereby producing the FGF8 and β-catenin markergene products. In some embodiments, the FGF8 mRNA is substantiallyexpressed in preprimitive streak cells but not in hESCs. Substantialupregulation of the FGF8 mRNA, to near peak levels, can be observed in adifferentiating hESC culture by 6 hours after contacting the hESCs withan appropriate differentiation factor, such as activin A. At this time,expression of markers indicative of other cells types, such asmesendoderm, primitive endoderm, definitive endoderm, mesoderm andectoderm (see Table 1), is still comparatively low. In some embodiments,markers indicative of mesendoderm, primitive endoderm, definitiveendoderm, mesoderm and ectoderm are not substantially expressed by 6hours after contacting the hESCs with the differentiation factor. FGF8mRNA expression is maintained at high levels for at least about 24 hoursafter contacting the hESCs with the differentiation factor but begins todecline thereafter. Additionally, by about 17 hours after contacting thehESCs with an appropriate differentiation factor, such as activin A,nuclear localization of the β-catenin polypeptide (the expression ofnuclear-localized β-catenin) is observed by immunocytochemistry. InhESCs, the β-catenin polypeptide is present at the cell periphery butnot in the nucleus.

It will be appreciated that FGF8 and nuclear-localized β-cateninexpression is induced over a range of different levels in preprimitivestreak cells depending on the differentiation conditions. As such, insome embodiments described herein, the expression of the FGF8 markerand/or the nuclear-localized β-catenin marker in preprimitive streakcells or cell populations is at least about 2-fold higher to at leastabout 10,000-fold higher than the expression of these markers innon-preprimitive streak cells or cell populations, during about thefirst 6 to 18 hours of differentiation from hESCs. In other embodiments,the expression of the FGF8 marker and/or the nuclear-localized β-cateninmarker in preprimitive streak cells or cell populations is at leastabout 4-fold higher, at least about 6-fold higher, at least about 8-foldhigher, at least about 10-fold higher, at least about 15-fold higher, atleast about 20-fold higher, at least about 40-fold higher, at leastabout 80-fold higher, at least about 100-fold higher, at least about150-fold higher, at least about 200-fold higher, at least about 500-foldhigher, at least about 750-fold higher, at least about 1000-fold higher,at least about 2500-fold higher, at least about 5000-fold higher, atleast about 7500-fold higher or at least about 10,000-fold higher thanthe expression of the FGF8 marker and/or the nuclear-localized β-cateninmarker in non-preprimitive streak cells or cell populations, duringabout the first 6 to 18 hours of differentiation from hESCs. In someembodiments, the expression of the FGF8 marker and/or thenuclear-localized β-catenin marker in preprimitive streak cells or cellpopulations is infinitely higher than the expression of the FGF8 markerand/or the nuclear-localized β-catenin marker in non-preprimitive streakcells or cell populations, during about the first 6 to 18 hours ofdifferentiation from hESCs.

Additionally, it will be appreciated that there is a range ofdifferences between the expression level of the FGF8 marker and theexpression levels of the brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7and/or SOX1 markers in preprimitive streak cells. Similarly, thereexists a range of differences between the expression level of thenuclear-localized β-catenin marker and the expression levels of thebrachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and/or SOX1 markers inpreprimitive streak cells. As such, in some embodiments describedherein, the expression of the FGF8 marker and/or the nuclear-localizedβ-catenin marker is at least about 2-fold higher to at least about10,000-fold higher than the expression of the brachyury, FGF4, SNAI1,SOX17, FOXA2, SOX7 and/or SOX1 markers. In other embodiments, theexpression of the FGF8 marker and/or the nuclear-localized β-cateninmarker is at least about 4-fold higher, at least about 6-fold higher, atleast about 8-fold higher, at least about 10-fold higher, at least about15-fold higher, at least about 20-fold higher, at least about 40-foldhigher, at least about 80-fold higher, at least about 100-fold higher,at least about 150-fold higher, at least about 200-fold higher, at leastabout 500-fold higher, at least about 750-fold higher, at least about1000-fold higher, at least about 2500-fold higher, at least about5000-fold higher, at least about 7500-fold higher or at least about10,000-fold higher than the expression of the brachyury, FGF4, SNAI1,SOX17, FOXA2, SOX7 and/or SOX1 markers. In some embodiments, thebrachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and/or SOX1 markers are notsignificantly expressed in preprimitive streak cells.

Enrichment, Isolation and/or Purification of Preprimitive Streak Cells

With respect to additional aspects of the processes described herein,preprimitive streak cells can be enriched, isolated and/or purified. Insome embodiments, cell populations enriched for preprimitive streakcells are produced by isolating such cells from cell cultures.

In some embodiments of the processes described herein, preprimitivestreak cells are fluorescently labeled then isolated from non-labeledcells by using a fluorescence activated cell sorter (FACS). In suchembodiments, a nucleic acid encoding green fluorescent protein (GFP) oranother nucleic acid encoding an expressible fluorescent marker gene isused to label preprimitive streak cells. For example, in someembodiments, at least one copy of a nucleic acid encoding GFP or abiologically active fragment thereof is introduced into a pluripotentcell, preferably a human embryonic stem cell, downstream of the FGF8promoter such that the expression of the GFP gene product orbiologically active fragment thereof is under control of the FGF8promoter. In some embodiments, the entire coding region of the nucleicacid, which encodes FGF8, is replaced by a nucleic acid encoding GFP ora biologically active fragment thereof. In other embodiments, thenucleic acid encoding GFP or a biologically active fragment thereof isfused in frame with at least a portion of the nucleic acid encodingFGF8, thereby generating a fusion protein. In such embodiments, thefusion protein retains a fluorescent activity similar to GFP.

Fluorescently marked cells, such as the above-described pluripotentcells, are differentiated to preprimitive streak cells as describedpreviously above. Because preprimitive streak cells express thefluorescent marker gene, whereas non-preprimitive streak cells do not,these two cell types can be separated. In some embodiments, cellsuspensions comprising a mixture of fluorescently-labeled preprimitivestreak cells and unlabeled non-preprimitive streak cells are sortedusing a FACS. Preprimitive streak cells are collected separately fromnon-preprimitive streak cells, thereby resulting in the isolation ofsuch cell types. If desired, the isolated cell compositions can befurther purified by additional rounds of sorting using the same ordifferent markers that are specific for preprimitive streak cells.

In addition to the procedures just described, preprimitive streak cellsmay also be isolated by other techniques for cell isolation.Additionally, preprimitive streak cells may also be enriched or isolatedby methods of serial subculture in growth conditions which promote theselective survival or selective expansion of the preprimitive streakcells.

It will be appreciated that the above-described enrichment, isolationand purification procedures can be used with such cultures at any stageof differentiation.

Using the methods described herein, enriched, isolated and/or purifiedpopulations of preprimitive streak cells and/or tissues can be producedin vitro from hESC cultures or cell populations which have undergonedifferentiation for from about 1 hour to about 24 hours. In someembodiments, the cells undergo random differentiation. In a preferredembodiment, however, the cells are directed to differentiate primarilyinto preprimitive streak cells. Some preferred enrichment, isolationand/or purification methods relate to the in vitro production ofpreprimitive streak cells from human embryonic stem cells.

Using the methods described herein, cell populations or cell culturescan be enriched in preprimitive streak cell content by at least about 2-to about 1000-fold as compared to untreated or unenriched cellpopulations or cell cultures. In some embodiments, preprimitive streakcells can be enriched by at least about 5- to about 500-fold as comparedto untreated or unenriched cell populations or cell cultures. In otherembodiments, preprimitive streak cells can be enriched from at leastabout 10- to about 200-fold as compared to untreated or unenriched cellpopulations or cell cultures. In still other embodiments, preprimitivestreak cells can be enriched from at least about 20- to about 100-foldas compared to untreated or unenriched cell populations or cellcultures. In yet other embodiments, preprimitive streak cells can beenriched from at least about 40- to about 80-fold as compared tountreated or unenriched cell populations or cell cultures. In certainembodiments, preprimitive streak cells can be enriched from at leastabout 2- to about 20-fold as compared to untreated or unenriched cellpopulations or cell cultures.

Compositions Comprising Preprimitive Streak Cells

Cell compositions produced by the above-described methods include cellcultures comprising preprimitive streak cells and cell populationsenriched in preprimitive streak cells. For example, cell cultures whichcomprise preprimitive streak cells, wherein at least about 5-90% of thecells in culture are preprimitive streak cells, can be produced. Becausethe efficiency of the differentiation process can be adjusted bymodifying certain parameters, which include but are not limited to, cellgrowth conditions, growth factor concentrations and the timing ofculture steps, the differentiation procedures described herein canresult in about 5%, about 10%, about 15%, about 20%, about 25%, about30%, about 35%, about 40%, about 45%, about 50%, about 55%, about 60%,about 65%, about 70%, about 75%, about 80%, about 85%, about 90%, about95%, or greater than about 95% conversion of pluripotent cells topreprimitive streak cells. In processes in which isolation ofpreprimitive streak cells is employed, a substantially pure preprimitivestreak cell population can be recovered.

Some embodiments described herein relate to compositions, such as cellpopulations and cell cultures, that comprise both pluripotent cells,such as stem cells, and preprimitive streak cells. For example, usingthe methods described herein, compositions comprising mixtures of hESCsand preprimitive streak cells can be produced. In some embodiments,compositions comprising at least about 5 preprimitive streak cells forabout every 95 pluripotent cells are produced. In other embodiments,compositions comprising at least about 95 preprimitive streak cells forabout every 5 pluripotent cells are produced. Additionally, compositionscomprising other ratios of preprimitive streak cells to pluripotentcells are contemplated. For example, compositions comprising at leastabout 1 preprimitive streak cell for about every 1,000,000 pluripotentcells, at least about 1 preprimitive streak cell for about every 100,000pluripotent cells, at least about 1 preprimitive streak cell for aboutevery 10,000 pluripotent cells, at least about 1 preprimitive streakcell for about every 1000 pluripotent cells, at least about 1preprimitive streak cell for about every 500 pluripotent cells, at leastabout 1 preprimitive streak cell for about every 100 pluripotent cells,at least about 1 preprimitive streak cell for about every 10 pluripotentcells, at least about 1 preprimitive streak cell for about every 5pluripotent cells, at least about 1 preprimitive streak cell for aboutevery 2 pluripotent cells, at least about 2 preprimitive streak cellsfor about every 1 pluripotent cell, at least about 5 preprimitive streakcells for about every 1 pluripotent cell, at least about 10 preprimitivestreak cells for about every 1 pluripotent cell, at least about 20preprimitive streak cells for about every 1 pluripotent cell, at leastabout 50 preprimitive streak cells for about every 1 pluripotent cell,at least about 100 preprimitive streak cells for about every 1pluripotent cell, at least about 1000 preprimitive streak cells forabout every 1 pluripotent cell, at least about 10,000 preprimitivestreak cells for about every 1 pluripotent cell, at least about 100,000preprimitive streak cells for about every 1 pluripotent cell and atleast about 1,000,000 preprimitive streak cells for about every 1pluripotent cell are contemplated. In some embodiments, the pluripotentcells are human pluripotent stem cells. In certain embodiments, the stemcells are derived from a morula, the inner cell mass of an embryo or thegonadal ridges of an embryo. In certain other embodiments, thepluripotent cells are derived from the gonadal or germ tissues of amulticellular structure that has developed past the embryonic stage.

Some embodiments described herein relate to cell cultures or cellpopulations comprising from at least about 5% preprimitive streak cellsto at least about 99% preprimitive streak cells. In some embodiments thecell cultures or cell populations comprise mammalian cells. In preferredembodiments, the cell cultures or cell populations comprise human cells.For example, certain specific embodiments relate to cell culturescomprising human cells, wherein from at least about 5% to at least about99% of the human cells are preprimitive streak cells. Other embodimentsrelate to cell cultures comprising human cells, wherein at least about5%, at least about 10%, at least about 15%, at least about 20%, at leastabout 25%, at least about 30%, at least about 35%, at least about 40%,at least about 45%, at least about 50%, at least about 55%, at leastabout 60%, at least about 65%, at least about 70%, at least about 75%,at least about 80%, at least about 85%, at least about 90%, at leastabout 95%, at least about 98%, at least about 99%, or greater than 99%of the human cells are preprimitive streak cells. In embodiments wherethe cell cultures or cell populations comprise human feeder cells, theabove percentages are calculated without respect to the human feedercells in the cell cultures or cell populations.

Further embodiments described herein relate to compositions, such ascell cultures or cell populations, comprising human cells, such as humanpreprimitive streak cells, wherein the expression of the FGF8 markerand/or the nuclear-localized (3-catenin marker is greater than theexpression of the brachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and/or SOX1markers in at least about 5% of the human cells. In other embodiments,the expression of either the FGF8 marker and/or the nuclear-localized(3-catenin marker is greater than the expression of the brachyury, FGF4,SNAI1, SOX17, FOXA2, SOX7 and/or SOX1 marker in at least about 10% ofthe human cells, in at least about 15% of the human cells, in at leastabout 20% of the human cells, in at least about 25% of the human cells,in at least about 30% of the human cells, in at least about 35% of thehuman cells, in at least about 40% of the human cells, in at least about45% of the human cells, in at least about 50% of the human cells, in atleast about 55% of the human cells, in at least about 60% of the humancells, in at least about 65% of the human cells, in at least about 70%of the human cells, in at least about 75% of the human cells, in atleast about 80% of the human cells, in at least about 85% of the humancells, in at least about 90% of the human cells, in at least about 95%of the human cells, in at least about 98% of the human cells, in atleast about 99% of the human cells or in greater than 99% of the humancells. In embodiments where the cell cultures or cell populationscomprise human feeder cells, the above percentages are calculatedwithout respect to the human feeder cells in the cell cultures or cellpopulations.

Additional embodiments described herein relate to compositions, such ascell cultures or cell populations, comprising human hESCs and humanpreprimitive streak cells, wherein substantial upregulation of theexpression of FGF8 mRNA occurs in cells of the cell culture or cellpopulation by about 1 hour, by about 2 hours, by about 3 hours, by about4 hours, by about 5 hours, by about 6 hours, by about 7 hours, by about8 hours, by about 9 hours, by about 10 hours, by about 11 hours, byabout 12 hours, by about 13 hours, by about 14 hours, by about 15 hours,by about 16 hours, by about 17 hours, by about 18 hours, by about 19hours, by about 20 hours, by about 21 hours, by about 22 hours, by about23 hours, by about 24 hours, or by greater than about 24 hours aftercontacting hESCs in the culture with an appropriate differentiationfactor, such as activin A. In such embodiments, substantial upregulationof the expression of FGF8 mRNA occurs in at least about 5% of the humancells, at least about 10% of the human cells, in at least about 15% ofthe human cells, in at least about 20% of the human cells, in at leastabout 25% of the human cells, in at least about 30% of the human cells,in at least about 35% of the human cells, in at least about 40% of thehuman cells, in at least about 45% of the human cells, in at least about50% of the human cells, in at least about 55% of the human cells, in atleast about 60% of the human cells, in at least about 65% of the humancells, in at least about 70% of the human cells, in at least about 75%of the human cells, in at least about 80% of the human cells, in atleast about 85% of the human cells, in at least about 90% of the humancells, in at least about 95% of the human cells, in at least about 98%of the human cells, in at least about 99% of the human cells or ingreater than 99% of the human cells. In embodiments where the cellcultures or cell populations comprise human feeder cells, the abovepercentages are calculated without respect to the human feeder cells inthe cell cultures or cell populations.

Further embodiments described herein relate to compositions, such ascell cultures or cell populations, comprising human hESCs and humanpreprimitive streak cells, wherein substantial nuclear localization ofthe β-catenin polypeptide (expression of nuclear localized β-cateninmarker) occurs in cells of the cell culture or cell population by about1 hour, by about 2 hours, by about 3 hours, by about 4 hours, by about 5hours, by about 6 hours, by about 7 hours, by about 8 hours, by about 9hours, by about 10 hours, by about 11 hours, by about 12 hours, by about13 hours, by about 14 hours, by about 15 hours, by about 16 hours, byabout 17 hours, by about 18 hours, by about 19 hours, by about 20 hours,by about 21 hours, by about 22 hours, by about 23 hours, by about 24hours, or by greater than about 24 hours after contacting hESCs in theculture with an appropriate differentiation factor, such as activin A.In such embodiments, expression of nuclear-localized β-catenin occurs inat least about 5% of the human cells, at least about 10% of the humancells, in at least about 15% of the human cells, in at least about 20%of the human cells, in at least about 25% of the human cells, in atleast about 30% of the human cells, in at least about 35% of the humancells, in at least about 40% of the human cells, in at least about 45%of the human cells, in at least about 50% of the human cells, in atleast about 55% of the human cells, in at least about 60% of the humancells, in at least about 65% of the human cells, in at least about 70%of the human cells, in at least about 75% of the human cells, in atleast about 80% of the human cells, in at least about 85% of the humancells, in at least about 90% of the human cells, in at least about 95%of the human cells, in at least about 98% of the human cells, in atleast about 99% of the human cells or in greater than 99% of the humancells. In embodiments where the cell cultures or cell populationscomprise human feeder cells, the above percentages are calculatedwithout respect to the human feeder cells in the cell cultures or cellpopulations.

Using the methods described herein, compositions comprising preprimitivestreak cells substantially free of other cell types can be produced. Insome embodiments described herein, the preprimitive streak cellpopulations or cell cultures produced by the methods described hereinare substantially free of cells that significantly express thebrachyury, FGF4, SNAI1, SOX17, FOXA2, SOX7 and/or SOX1 marker genes.

In one embodiment, a description of a preprimitive streak cell based onthe expression of marker genes is, FGF8 high, nuclear-localizedβ-catenin high, brachyury low, FGF4 low, SNAI1 low, SOX17 low, FOXA2low, SOX7 low and SOX1 low.

Production of Mesendoderm Cells from Pluripotent Cells

In some processes for producing mesendoderm cells, the pluripotent cellsused as starting material are stem cells. In certain processes,mesendoderm cell cultures and enriched cell populations comprisingmesendoderm cells are produced from embryonic stem cells. A preferredmethod for deriving mesendoderm cells utilizes human embryonic stemcells as the starting material for mesendoderm cell production. Suchpluripotent cells can be cells that originate from the morula, embryonicinner cell mass or those obtained from embryonic gonadal ridges. Humanembryonic stem cells can be maintained in culture in a pluripotent statewithout substantial differentiation using methods that are known in theart. Such methods are described, for example, in U.S. Pat. Nos.5,453,357, 5,670,372, 5,690,926 5,843,780, 6,200,806 and 6,251,671 thedisclosures of which are incorporated herein by reference in theirentireties.

In some processes for producing mesendoderm cells, hESCs are maintainedon a feeder layer. In such processes, any feeder layer which allowshESCs to be maintained in a pluripotent state can be used. One commonlyused feeder layer for the cultivation of human embryonic stem cells is alayer of mouse fibroblasts. More recently, human fibroblast feederlayers have been developed for use in the cultivation of hESCs (see USPatent Application No. 2002/0072117, the disclosure of which isincorporated herein by reference in its entirety). Alternative processesfor producing mesendoderm cells permit the maintenance of pluripotenthESC without the use of a feeder layer. Methods of maintainingpluripotent hESCs under feeder-free conditions have been described in USPatent Application No. 2003/0175956, the disclosure of which isincorporated herein by reference in its entirety.

The human embryonic stem cells used herein can be maintained in cultureeither with or without serum. In some embryonic stem cell maintenanceprocedures, serum replacement is used. In others, serum free culturetechniques, such as those described in US Patent Application No.2003/0190748, the disclosure of which is incorporated herein byreference in its entirety, are used.

Stem cells are maintained in culture in a pluripotent state by routinepassage until it is desired that they be differentiated into mesendodermcells. In some processes, differentiation to mesendoderm cells isachieved by providing to the stem cell culture a differentiation factor,such as a growth factor of the TGFβ superfamily, in an amount sufficientto promote differentiation to mesendoderm cells. Growth factors of theTGFβ superfamily which are useful for the production of mesendodermcells are selected from the Nodal/Activin subgroups. In some preferreddifferentiation processes, the growth factor is selected from the groupconsisting of Nodal, activin A, and activin B. In certaindifferentiation processes, the growth factor activin A is used.

With respect to some of the processes for the differentiation ofpluripotent stem cells to mesendoderm cells, the above-mentioned growthfactors are provided to the cells so that the growth factors are presentin the cultures at concentrations sufficient to promote differentiationof at least a portion of the stem cells to mesendoderm cells. In someprocesses, the above-mentioned growth factors are present in the cellculture at a concentration of at least about 5 ng/ml, at least about 10ng/ml, at least about 25 ng/ml, at least about 50 ng/ml, at least about75 ng/ml, at least about 100 ng/ml, at least about 200 ng/ml, at leastabout 300 ng/ml, at least about 400 ng/ml, at least about 500 ng/ml, atleast about 1000 ng/ml, at least about 2000 ng/ml, at least about 3000ng/ml, at least about 4000 ng/ml, at least about 5000 ng/ml or more thanabout 5000 ng/ml.

In certain processes for the differentiation of pluripotent stem cellsto mesendoderm cells, the above-mentioned growth factors are removedfrom the cell culture subsequent to their addition. For example, thegrowth factors can be removed within about 18 hours, within about 19hours, within about 20 hours, within about 21 hours, within about 22hours, within about 23 hours, within about 24 hours, within about 25hours, within about 26 hours, within about 27 hours, within about 28hours, within about 29 hours, within about 30 hours, within about 31hours, within about 32 hours, within about 33 hours, within about 34hours, within about 35 hours, within about 36 hours, within about 37hours, within about 38 hours, within about 39 hours, within about 40hours, within about 41 hours, within about 42 hours, within about 43hours, within about 44 hours, within about 45 hours, within about 46hours, within about 47 hours, within about 48 hours or within about morethan 48 hours.

Cultures of mesendoderm cells can be grown in medium containing reducedserum or no serum. Under certain culture conditions, serumconcentrations can range from about 0% (v/v) to about 10% (v/v). Forexample, in some differentiation processes, the serum concentration ofthe medium can be less than about 0.05% (v/v), less than about 0.1%(v/v), less than about 0.2% (v/v), less than about 0.3% (v/v), less thanabout 0.4% (v/v), less than about 0.5% (v/v), less than about 0.6%(v/v), less than about 0.7% (v/v), less than about 0.8% (v/v), less thanabout 0.9% (v/v), less than about 1% (v/v), less than about 2% (v/v),less than about 3% (v/v), less than about 4% (v/v), less than about 5%(v/v), less than about 6% (v/v), less than about 7% (v/v), less thanabout 8% (v/v), less than about 9% (v/v) or less than about 10% (v/v).In some processes, mesendoderm cells are grown without serum or withoutserum replacement. In still other processes, mesendoderm cells are grownin the presence of B27. In such processes, the concentration of B27supplement can range from about 0.1% (v/v) to about 20% (v/v).

Monitoring the Differentiation of Pluripotent Cells to Mesendoderm Cells

The progression of the hESC culture to mesendoderm cells can bemonitored by determining the temporal expression of markerscharacteristic of mesendoderm cells. In some processes, the expressionof certain markers is determined by detecting the presence or absence ofthe marker. Alternatively, the expression of certain markers can bedetermined by measuring the level at which the marker is present in thecells of the cell culture or cell population at one or more time pointssubsequent to the addition of the differentiation factor. In suchprocesses, the measurement of marker expression can be qualitative orquantitative. One method of quantitating the expression of markers thatare produced by marker genes is through the use of Q-PCR. Other methodswhich are known in the art can also be used to quantitate marker geneexpression. For example, the expression of a marker gene product can bedetected by using antibodies specific for the marker gene product ofinterest. In certain processes, the expression of marker genescharacteristic of mesendoderm cells as well as the lack of significantexpression of marker genes characteristic of hESCs and other cell typesis determined. In still other processes, both the timing and amount ofexpression of marker genes characteristic of mesendoderm cells at one ormore time points subsequent to the addition of the differentiationfactor is determined.

As described further in the Examples below, markers of mesendoderm cellsare brachyury, FGF4 and SNAI1. As such, the mesendoderm cells producedby the processes described herein express the brachyury, FGF4 and SNAI1marker genes, thereby producing the brachyury, FGF4 and SNAI1 markergene products. In some embodiments, the brachyury, FGF4 and/or SNAI1mRNA is substantially expressed in mesendoderm cells but not in hESCs.Substantial upregulation of the brachyury, FGF4 and/or SNAI1 mRNA, tonear peak levels, can be observed in a differentiating hESC culture by24 hours after contacting the hESCs with an appropriate differentiationfactor, such as activin A. At this time, expression of certain markersindicative of other cells types, such as primitive endoderm, definitiveendoderm, mesoderm and ectoderm (see Table 1), is still comparativelylow. In some embodiments, certain markers indicative of primitiveendoderm, definitive endoderm, mesoderm and ectoderm are notsubstantially expressed by 24 hours after contacting the hESCs with thedifferentiation factor. In some embodiments, brachyury, FGF4 and/orSNAI1 mRNA expression begins to decline after about 24 hours subsequentto contacting the hESCs with the differentiation factor. In someembodiments, brachyury, FGF4 and/or SNAI1 mRNA expression begins todecline after about 30 hours, after about 36 hours, after about 42hours, after about 48 hours or after more than about 48 hours subsequentto contacting the hESCs with the differentiation factor.

It will be appreciated that brachyury, FGF4 and/or SNAI1 expression isinduced over a range of different levels in mesendoderm cells dependingon the differentiation conditions. As such, in some embodimentsdescribed herein, the expression of the brachyury, FGF4 and/or SNAI1marker in mesendoderm cells or cell populations is at least about 2-foldhigher to at least about 10,000-fold higher than the expression of thesemarkers in non-mesendoderm cells or cell populations, after about 24hours of differentiation from hESCs. In other embodiments, theexpression of the brachyury, FGF4 and/or SNAI1 marker in mesendodermcells or cell populations is at least about 4-fold higher, at leastabout 6-fold higher, at least about 8-fold higher, at least about10-fold higher, at least about 15-fold higher, at least about 20-foldhigher, at least about 40-fold higher, at least about 80-fold higher, atleast about 100-fold higher, at least about 150-fold higher, at leastabout 200-fold higher, at least about 500-fold higher, at least about750-fold higher, at least about 1000-fold higher, at least about2500-fold higher, at least about 5000-fold higher, at least about7500-fold higher or at least about 10,000-fold higher than theexpression of the brachyury, FGF4 and/or SNAI1 in non-mesendoderm cellsor cell populations, after about 24 hours of differentiation from hESCs.In some embodiments, the expression of the brachyury, FGF4 and/or SNAI1marker in mesendoderm cells or cell populations is infinitely higherthan the expression of the brachyury, FGF4 and/or SNAI1 marker innon-mesendoderm cells or cell populations, after about 24 hours ofdifferentiation from hESCs.

Additionally, it will be appreciated that there is a range ofdifferences between the expression level of the brachyury, FGF4 and/orSNAI1 markers and the expression levels of the OCT4, SOX17, CXCR4,FOXA2, SOX7 and/or SOX1 markers in mesendoderm cells. As such, in someembodiments described herein, the expression of the brachyury, FGF4and/or SNAI1 markers is at least about 2-fold higher to at least about10,000-fold higher than the expression of the OCT4, SOX17, CXCR4, FOXA2,SOX7 and/or SOX1 markers. In other embodiments, the expression of thebrachyury, FGF4 and/or SNAI1 marker is at least about 4-fold higher, atleast about 6-fold higher, at least about 8-fold higher, at least about10-fold higher, at least about 15-fold higher, at least about 20-foldhigher, at least about 40-fold higher, at least about 80-fold higher, atleast about 100-fold higher, at least about 150-fold higher, at leastabout 200-fold higher, at least about 500-fold higher, at least about750-fold higher, at least about 1000-fold higher, at least about2500-fold higher, at least about 5000-fold higher, at least about7500-fold higher or at least about 10,000-fold higher than theexpression of the OCT4, SOX17, CXCR4, FOXA2, SOX7 and/or SOX1 markers.In some embodiments, the OCT4, SOX17, CXCR4, FOXA2, SOX7 and/or SOX1markers are not significantly expressed in mesendoderm cells.

Enrichment Isolation and/or Purification of Mesendoderm Cells

With respect to additional aspects of the processes described herein,mesendoderm cells can be enriched, isolated and/or purified. In someembodiments, cell populations enriched for mesendoderm cells areproduced by isolating such cells from cell cultures.

In some embodiments of the processes described herein, mesendoderm cellsare fluorescently labeled then isolated from non-labeled cells by usinga FACS. In such embodiments, a nucleic acid encoding green fluorescentprotein (GFP) or another nucleic acid encoding an expressiblefluorescent marker gene is used to label mesendoderm cells. For example,in some embodiments, at least one copy of a nucleic acid encoding GFP ora biologically active fragment thereof is introduced into a pluripotentcell, preferably a human embryonic stem cell, downstream of thebrachyury, FGF4 or SNAI1 promoter such that the expression of the GFPgene product or biologically active fragment thereof is under control ofthe brachyury, FGF4 or SNAI1 promoter. In some embodiments, the entirecoding region of the nucleic acid, which encodes brachyury, FGF4 orSNAI1, is replaced by a nucleic acid encoding GFP or a biologicallyactive fragment thereof. In other embodiments, the nucleic acid encodingGFP or a biologically active fragment thereof is fused in frame with atleast a portion of the nucleic acid encoding brachyury, FGF4 or SNAI1thereby generating a fusion protein. In such embodiments, the fusionprotein retains a fluorescent activity similar to GFP.

Fluorescently marked cells, such as the above-described pluripotentcells, are differentiated to mesendoderm cells as described previouslyabove. Because mesendoderm cells express the fluorescent marker gene,whereas non-mesendoderm cells do not, these two cell types can beseparated. In some embodiments, cell suspensions comprising a mixture offluorescently-labeled mesendoderm cells and unlabeled non-mesendodermcells are sorted using a FACS. Mesendoderm cells are collectedseparately from non-mesendoderm cells, thereby resulting in theisolation of such cell types. If desired, the isolated cell compositionscan be further purified by additional rounds of sorting using the sameor different markers that are specific for mesendoderm cells.

In addition to the procedures just described, mesendoderm cells may alsobe isolated by other techniques for cell isolation. Additionally,mesendoderm cells may also be enriched or isolated by methods of serialsubculture in growth conditions which promote the selective survival orselective expansion of the mesendoderm cells.

It will be appreciated that the above-described enrichment, isolationand purification procedures can be used with such cultures at any stageof differentiation.

Using the methods described herein, enriched, isolated and/or purifiedpopulations of mesendoderm cells and/or tissues can be produced in vitrofrom hESC cultures or cell populations which have undergonedifferentiation for from about 18 hours to about 48 hours. In someembodiments, the cells undergo random differentiation. In a preferredembodiment, however, the cells are directed to differentiate primarilyinto mesendoderm cells. Some preferred enrichment, isolation and/orpurification methods relate to the in vitro production of mesendodermcells from human embryonic stem cells.

Using the methods described herein, cell populations or cell culturescan be enriched in mesendoderm cell content by at least about 2- toabout 1000-fold as compared to untreated or unenriched cell populationsor cell cultures. In some embodiments, mesendoderm cells can be enrichedby at least about 5- to about 500-fold as compared to untreated orunenriched cell populations or cell cultures. In other embodiments,mesendoderm cells can be enriched from at least about 10- to about200-fold as compared to untreated or unenriched cell populations or cellcultures. In still other embodiments, mesendoderm cells can be enrichedfrom at least about 20- to about 100-fold as compared to untreated orunenriched cell populations or cell cultures. In yet other embodiments,mesendoderm cells can be enriched from at least about 40- to about80-fold as compared to untreated or unenriched cell populations or cellcultures. In certain embodiments, mesendoderm cells can be enriched fromat least about 2- to about 20-fold as compared to untreated orunenriched cell populations or cell cultures.

Compositions Comprising Mesendoderm Cells

Cell compositions produced by the above-described methods include cellcultures comprising mesendoderm cells and cell populations enriched inmesendoderm cells. For example, cell cultures which comprise mesendodermcells, wherein at least about 5-90% of the cells in culture aremesendoderm cells, can be produced. Because the efficiency of thedifferentiation process can be adjusted by modifying certain parameters,which include but are not limited to, cell growth conditions, growthfactor concentrations and the timing of culture steps, thedifferentiation procedures described herein can result in about 5%,about 10%, about 15%, about 20%, about 25%, about 30%, about 35%, about40%, about 45%, about 50%, about 55%, about 60%, about 65%, about 70%,about 75%, about 80%, about 85%, about 90%, about 95%, or greater thanabout 95% conversion of pluripotent cells to mesendoderm cells. Inprocesses in which isolation of mesendoderm cells is employed, asubstantially pure mesendoderm cell population can be recovered.

Some embodiments described herein relate to compositions, such as cellpopulations and cell cultures, that comprise both pluripotent cells,such as stem cells, and mesendoderm cells. For example, using themethods described herein, compositions comprising mixtures of hESCs andmesendoderm cells can be produced. In some embodiments, compositionscomprising at least about 5 mesendoderm cells for about every 95pluripotent cells are produced. In other embodiments, compositionscomprising at least about 95 mesendoderm cells for about every 5pluripotent cells are produced. Additionally, compositions comprisingother ratios of mesendoderm cells to pluripotent cells are contemplated.For example, compositions comprising at least about 1 mesendoderm cellfor about every 1,000,000 pluripotent cells, at least about 1mesendoderm cell for about every 100,000 pluripotent cells, at leastabout 1 mesendoderm cell for about every 10,000 pluripotent cells, atleast about 1 mesendoderm cell for about every 1000 pluripotent cells,at least about 1 mesendoderm cell for about every 500 pluripotent cells,at least about 1 mesendoderm cell for about every 100 pluripotent cells,at least about 1 mesendoderm cell for about every 10 pluripotent cells,at least about 1 mesendoderm cell for about every 5 pluripotent cells,at least about 1 mesendoderm cell for about every 2 pluripotent cells,at least about 2 mesendoderm cells for about every 1 pluripotent cell,at least about 5 mesendoderm cells for about every 1 pluripotent cell,at least about 10 mesendoderm cells for about every 1 pluripotent cell,at least about 20 mesendoderm cells for about every 1 pluripotent cell,at least about 50 mesendoderm cells for about every 1 pluripotent cell,at least about 100 mesendoderm cells for about every 1 pluripotent cell,at least about 1000 mesendoderm cells for about every 1 pluripotentcell, at least about 10,000 mesendoderm cells for about every 1pluripotent cell, at least about 100,000 mesendoderm cells for aboutevery 1 pluripotent cell and at least about 1,000,000 mesendoderm cellsfor about every 1 pluripotent cell are contemplated. In someembodiments, the pluripotent cells are human pluripotent stem cells. Incertain embodiments, the stem cells are derived from a morula, the innercell mass of an embryo or the gonadal ridges of an embryo. In certainother embodiments, the pluripotent cells are derived from the gonadal orgerm tissues of a multicellular structure that has developed past theembryonic stage.

Some embodiments described herein relate to cell cultures or cellpopulations comprising from at least about 5% mesendoderm cells to atleast about 99% mesendoderm cells. In some embodiments the cell culturesor cell populations comprise mammalian cells. In preferred embodiments,the cell cultures or cell populations comprise human cells. For example,certain specific embodiments relate to cell cultures comprising humancells, wherein from at least about 5% to at least about 99% of the humancells are mesendoderm cells. Other embodiments relate to cell culturescomprising human cells, wherein at least about 5%, at least about 10%,at least about 15%, at least about 20%, at least about 25%, at leastabout 30%, at least about 35%, at least about 40%, at least about 45%,at least about 50%, at least about 55%, at least about 60%, at leastabout 65%, at least about 70%, at least about 75%, at least about 80%,at least about 85%, at least about 90%, at least about 95%, at leastabout 98%, at least about 99%, or greater than 99% of the human cellsare mesendoderm cells. In embodiments where the cell cultures or cellpopulations comprise human feeder cells, the above percentages arecalculated without respect to the human feeder cells in the cellcultures or cell populations.

Further embodiments described herein relate to compositions, such ascell cultures or cell populations, comprising human cells, such as humanmesendoderm cells, wherein the expression of the brachyury, FGF4 and/orSNAI1 markers is greater than the expression of the OCT4, SOX17, CXCR4,FOXA2, SOX7 and/or SOX1 markers in at least about 5% of the human cells.In other embodiments, the expression of either the brachyury, FGF4and/or SNAI1 marker is greater than the expression of the OCT4, SOX17,CXCR4, FOXA2, SOX7 and/or SOX1 marker in at least about 10% of the humancells, in at least about 15% of the human cells, in at least about 20%of the human cells, in at least about 25% of the human cells, in atleast about 30% of the human cells, in at least about 35% of the humancells, in at least about 40% of the human cells, in at least about 45%of the human cells, in at least about 50% of the human cells, in atleast about 55% of the human cells, in at least about 60% of the humancells, in at least about 65% of the human cells, in at least about 70%of the human cells, in at least about 75% of the human cells, in atleast about 80% of the human cells, in at least about 85% of the humancells, in at least about 90% of the human cells, in at least about 95%of the human cells, in at least about 98% of the human cells, in atleast about 99% of the human cells or in greater than 99% of the humancells. In embodiments where the cell cultures or cell populationscomprise human feeder cells, the above percentages are calculatedwithout respect to the human feeder cells in the cell cultures or cellpopulations.

Additional embodiments described herein relate to compositions, such ascell cultures or cell populations, comprising human hESCs and humanmesendoderm cells, wherein substantial upregulation of the expression ofbrachyury, FGF4 and/or SNAI1 mRNA occurs in cells of the cell culture orcell population by about 18 hours, by about 19 hours, by about 20 hours,by about 21 hours, by about 22 hours, by about 23 hours, by about 24hours, by about 25 hours, by about 26 hours, by about 27 hours, by about28 hours, by about 29 hours, by about 30 hours, by about 31 hours, byabout 32 hours, by about 33 hours, by about 34 hours, by about 35 hours,by about 36 hours, by about 37 hours, by about 38 hours, by about 39hours, by about 40 hours, by about 41 hours, by about 42 hours, by about43 hours, by about 44 hours, by about 45 hours, by about 46 hours, byabout 47 hours, by about 48 hours or by greater than about 48 hoursafter contacting hESCs in the culture with an appropriatedifferentiation factor, such as activin A. In such embodiments,substantial upregulation of the expression of brachyury, FGF4 and/orSNAI1 mRNA occurs in at least about 5% of the human cells, at leastabout 10% of the human cells, in at least about 15% of the human cells,in at least about 20% of the human cells, in at least about 25% of thehuman cells, in at least about 30% of the human cells, in at least about35% of the human cells, in at least about 40% of the human cells, in atleast about 45% of the human cells, in at least about 50% of the humancells, in at least about 55% of the human cells, in at least about 60%of the human cells, in at least about 65% of the human cells, in atleast about 70% of the human cells, in at least about 75% of the humancells, in at least about 80% of the human cells, in at least about 85%of the human cells, in at least about 90% of the human cells, in atleast about 95% of the human cells, in at least about 98% of the humancells, in at least about 99% of the human cells or in greater than 99%of the human cells. In embodiments where the cell cultures or cellpopulations comprise human feeder cells, the above percentages arecalculated without respect to the human feeder cells in the cellcultures or cell populations.

Using the methods described herein, compositions comprising mesendodermcells substantially free of other cell types can be produced. In someembodiments described herein, the mesendoderm cell populations or cellcultures produced by the methods described herein are substantially freeof cells that significantly express the OCT4, SOX17, CXCR4, FOXA2, SOX7and/or SOX1 marker genes.

In one embodiment, a description of a mesendoderm cell based on theexpression of marker genes is, brachyury high, FGF4 high, SNAI1 high,SOX17 low, CXCR4 low, FOXA2 low, SOX7 low and SOX1 low.

Production of Definitive Endoderm Cells

Processes for differentiating pluripotent cells to produce cell culturesand enriched cell populations comprising definitive endoderm isdescribed briefly below and in detail in U.S. patent Ser. No.11/021,618, entitled DEFINITIVE ENDODERM, filed Dec. 23, 2004, thedisclosure of which is incorporated herein by reference in its entirety.In some of these processes, the pluripotent cells used as startingmaterial are stem cells. In certain processes, definitive endoderm cellcultures and enriched cell populations comprising definitive endodermcells are produced from embryonic stem cells. A preferred method forderiving definitive endoderm cells utilizes human embryonic stem cellsas the starting material for definitive endoderm production. Suchpluripotent cells can be cells that originate from the morula, embryonicinner cell mass or those obtained from embryonic gonadal ridges. Humanembryonic stem cells can be maintained in culture in a pluripotent statewithout substantial differentiation using methods that are known in theart. Such methods are described, for example, in U.S. Pat. Nos.5,453,357, 5,670,372, 5,690,926 5,843,780, 6,200,806 and 6,251,671 thedisclosures of which are incorporated herein by reference in theirentireties.

In some processes for producing definitive endoderm cells, hESCs aremaintained on a feeder layer. In such processes, any feeder layer whichallows hESCs to be maintained in a pluripotent state can be used. Onecommonly used feeder layer for the cultivation of human embryonic stemcells is a layer of mouse fibroblasts. More recently, human fibroblastfeeder layers have been developed for use in the cultivation of hESCs(see US Patent Application No. 2002/0072117, the disclosure of which isincorporated herein by reference in its entirety). Alternative processesfor producing definitive endoderm permit the maintenance of pluripotenthESC without the use of a feeder layer. Methods of maintainingpluripotent hESCs under feeder-free conditions have been described in USPatent Application No. 2003/0175956, the disclosure of which isincorporated herein by reference in its entirety.

The human embryonic stem cells used herein can be maintained in cultureeither with or without serum. In some embryonic stem cell maintenanceprocedures, serum replacement is used. In others, serum free culturetechniques, such as those described in US Patent Application No.2003/0190748, the disclosure of which is incorporated herein byreference in its entirety, are used.

Stem cells are maintained in culture in a pluripotent state by routinepassage until it is desired that they be differentiated into definitiveendoderm. In some processes, differentiation to definitive endoderm isachieved by providing to the stem cell culture a growth factor of theTGFβ superfamily in an amount sufficient to promote differentiation todefinitive endoderm. Growth factors of the TGFβ superfamily which areuseful for the production of definitive endoderm are selected from theNodal/Activin subgroups. In some preferred differentiation processes,the growth factor is selected from the group consisting of Nodal,Activin A and Activin B. In certain differentiation processes,combinations of any of the above-mentioned growth factors can be used.

With respect to some of the processes for the differentiation ofpluripotent stem cells to definitive endoderm cells, the above-mentionedgrowth factors are provided to the cells so that the growth factors arepresent in the cultures at concentrations sufficient to promotedifferentiation of at least a portion of the stem cells to definitiveendoderm cells. In some processes, the above-mentioned growth factorsare present in the cell culture at a concentration of at least about 5ng/ml, at least about 10 ng/ml, at least about 25 ng/ml, at least about50 ng/ml, at least about 75 ng/ml, at least about 100 ng/ml, at leastabout 200 ng/ml, at least about 300 ng/ml, at least about 400 ng/ml, atleast about 500 ng/ml, at least about 1000 ng/ml, at least about 2000ng/ml, at least about 3000 ng/ml, at least about 4000 ng/ml, at leastabout 5000 ng/ml or more than about 5000 ng/ml.

In certain processes for the differentiation of pluripotent stem cellsto definitive endoderm cells, the above-mentioned growth factors areremoved from the cell culture subsequent to their addition. For example,the growth factors can be removed within about one day, about two days,about three days, about four days, about five days, about six days,about seven days, about eight days, about nine days or about ten daysafter their addition. In a preferred processes, the growth factors areremoved about four days after their addition.

Cultures of definitive endoderm cells can be grown in medium containingreduced serum or no serum. Under certain culture conditions, serumconcentrations can range from about 0.05% v/v to about 20% v/v. Forexample, in some differentiation processes, the serum concentration ofthe medium can be less than about 0.05% (v/v), less than about 0.1%(v/v), less than about 0.2% (v/v), less than about 0.3% (v/v), less thanabout 0.4% (v/v), less than about 0.5% (v/v), less than about 0.6%(v/v), less than about 0.7% (v/v), less than about 0.8% (v/v), less thanabout 0.9% (v/v), less than about 1% (v/v), less than about 2% (v/v),less than about 3% (v/v), less than about 4% (v/v), less than about 5%(v/v), less than about 6% (v/v), less than about 7% (v/v), less thanabout 8% (v/v), less than about 9% (v/v), less than about 10% (v/v),less than about 15% (v/v) or less than about 20% (v/v). In someprocesses, definitive endoderm cells are grown without serum or withoutserum replacement. In some embodiments, definitive endoderm cells aregrown with serum replacement. In still other processes, definitiveendoderm cells are grown in the presence of B27. In such processes, theconcentration of B27 supplement can range from about 0.1% v/v to about20% v/v.

Monitoring the Differentiation of Pluripotent Cells to DefinitiveEndoderm Cells

The progression of the hESC culture to definitive endoderm can bemonitored by determining the expression of markers characteristic ofdefinitive endoderm. In some processes, the expression of certainmarkers is determined by detecting the presence or absence of themarker. Alternatively, the expression of certain markers can determinedby measuring the level at which the marker is present in the cells ofthe cell culture or cell population. In such processes, the measurementof marker expression can be qualitative or quantitative. One method ofquantitating the expression of markers that are produced by marker genesis through the use of Q-PCR. Other methods which are known in the artcan also be used to quantitate marker gene expression. For example, theexpression of a marker gene product can be detected by using antibodiesspecific for the marker gene product of interest. In certain processes,the expression of marker genes characteristic of definitive endoderm aswell as the lack of significant expression of marker genescharacteristic of hESCs and other cell types is determined.

As described further in the Examples below, a reliable marker ofdefinitive endoderm is the SOX17 gene. As such, the definitive endodermcells produced by the processes described herein express the SOX17marker gene, thereby producing the SOX17 gene product. Other markers ofdefinitive endoderm are MIXL1, GATA4, HNF3b, GSC, FGF17, VWF, CALCR,FOXQ1, CMKOR1 and CRIP1. Since definitive endoderm cells express theSOX17 marker gene at a level higher than that of the SOX7 marker gene,which is characteristic of primitive and visceral endoderm, in someprocesses, the expression of both SOX17 and SOX7 is monitored. In otherprocesses, expression of the both the SOX17 marker gene and the OCT4marker gene, which is characteristic of hESCs, is monitored.Additionally, because definitive endoderm cells express the SOX17 markergene at a level higher than that of the AFP, SPARC or Thrombomodulin™marker genes, the expression of these genes can also be monitored.

Another marker of definitive endoderm is the CXCR4 gene. The CXCR4 geneencodes a cell surface chemokine receptor whose ligand is thechemoattractant SDF-1. The principal roles of the CXCR4 receptor-bearingcells in the adult are believed to be the migration of hematopoeticcells to the bone marrow, lymphocyte trafficking and the differentiationof various B cell and macrophage blood cell lineages [Kim, C., andBroxmeyer, H. J. Leukocyte Biol. 65, 6-15 (1999)]. The CXCR4 receptoralso functions as a coreceptor for the entry of HIV-1 into T-cells[Feng, Y., et al. Science, 272, 872-877 (1996)]. In an extensive seriesof studies carried out by [McGrath, K. E. et al. Dev. Biology 213,442-456 (1999)], the expression of the chemokine receptor CXCR4 and itsunique ligand, SDF-1 [Kim, C., and Broxmyer, H., J. Leukocyte Biol. 65,6-15 (1999)], were delineated during early development and adult life inthe mouse. The CXCR4/SDF1 interaction in development became apparentwhen it was demonstrated that if either gene was disrupted in transgenicmice [Nagasawa et al. Nature, 382, 635-638 (1996)], Ma, Q., et alImmunity, 10, 463-471 (1999)] it resulted in late embryonic lethality.McGrath et al. demonstrated that CXCR4 is the most abundant chemokinereceptor messenger RNA detected during early gastrulating embryos (E7.5)using a combination of RNase protection and in situ hybridizationmethodologies. In the gastrulating embryo, CXCR4/SDF-1 signaling appearsto be mainly involved in inducing migration of primitive-streakgermlayer cells and is expressed on definitive endoderm, mesoderm andextraembryonic mesoderm present at this time. In E7.2-7.8 mouse embryos,CXCR4 and alpha-fetoprotein are mutually exclusive indicating a lack ofexpression in visceral endoderm [McGrath, K. E. et al. Dev. Biology 213,442-456 (1999)].

Since definitive endoderm cells produced by differentiating pluripotentcells express the CXCR4 marker gene, expression of CXCR4 can bemonitored in order to track the production of definitive endoderm cells.Additionally, definitive endoderm cells produced by the methodsdescribed herein express other markers of definitive endoderm including,but not limited to, SOX17, MIXL1, GATA4, HNF3b, GSC, FGF17, VWF, CALCR,FOXQ1, CMKOR1 and CRIP1. Since definitive endoderm cells express theCXCR4 marker gene at a level higher than that of the SOX7 marker gene,the expression of both CXCR4 and SOX7 can be monitored. In otherprocesses, expression of the both the CXCR4 marker gene and the OCT4marker gene, is monitored. Additionally, because definitive endodermcells express the CXCR4 marker gene at a level higher than that of theAFP, SPARC or Thrombomodulin™ marker genes, the expression of thesegenes can also be monitored.

It will be appreciated that expression of CXCR4 in endodermal cells doesnot preclude the expression of SOX17. As such, definitive endoderm cellsproduced by the processes described herein will substantially expressSOX17 and CXCR4 but will not substantially express AFP, TM, SPARC orPDX1.

Enrichment, Isolation and/or Purification of Definitive Endoderm Cells

Definitive endoderm cells produced by any of the above-describedprocesses can be enriched, isolated and/or purified by using an affinitytag that is specific for such cells. Examples of affinity tags specificfor definitive endoderm cells are antibodies, ligands or other bindingagents that are specific to a marker molecule, such as a polypeptide,that is present on the cell surface of definitive endoderm cells butwhich is not substantially present on other cell types that would befound in a cell culture produced by the methods described herein. Insome processes, an antibody which binds to CXCR4 is used as an affinitytag for the enrichment, isolation or purification of definitive endodermcells. In other processes, the chemokine SDF-1 or other molecules basedon SDF-1 can also be used as affinity tags. Such molecules include, butnot limited to, SDF-1 fragments, SDF-1 fusions or SDF-1 mimetics.

Methods for making antibodies and using them for cell isolation areknown in the art and such methods can be implemented for use with theantibodies and definitive endoderm cells described herein. In oneprocess, an antibody which binds to CXCR4 is attached to a magnetic beadand then allowed to bind to definitive endoderm cells in a cell culturewhich has been enzymatically treated to reduce intercellular andsubstrate adhesion. The cell/antibody/bead complexes are then exposed toa movable magnetic field which is used to separate bead-bound definitiveendoderm cells from unbound cells. Once the definitive endoderm cellsare physically separated from other cells in culture, the antibodybinding is disrupted and the cells are replated in appropriate tissueculture medium.

Additional methods for obtaining enriched, isolated or purifieddefinitive endoderm cell cultures or populations can also be used. Forexample, in some embodiments, the CXCR4 antibody is incubated with adefinitive endoderm-containing cell culture that has been treated toreduce intercellular and substrate adhesion. The cells are then washed,centrifuged and resuspended. The cell suspension is then incubated witha secondary antibody, such as an FITC-conjugated antibody that iscapable of binding to the primary antibody. The cells are then washed,centrifuged and resuspended in buffer. The cell suspension is thenanalyzed and sorted using a fluorescence activated cell sorter (FACS).CXCR4-positive cells are collected separately from CXCR4-negative cells,thereby resulting in the isolation of such cell types. If desired, theisolated cell compositions can be further purified by using an alternateaffinity-based method or by additional rounds of sorting using the sameor different markers that are specific for definitive endoderm.

In still other processes, definitive endoderm cells are enriched,isolated and/or purified using a ligand or other molecule that binds toCXCR4. In some processes, the molecule is SDF-1 or a fragment, fusion ormimetic thereof.

In preferred processes, definitive endoderm cells are enriched, isolatedand/or purified from other non-definitive endoderm cells after the stemcell cultures are induced to differentiate towards the definitiveendoderm lineage. It will be appreciated that the above-describedenrichment, isolation and purification procedures can be used with suchcultures at any stage of differentiation.

In addition to the procedures just described, definitive endoderm cellsmay also be isolated by other techniques for cell isolation.Additionally, definitive endoderm cells may also be enriched or isolatedby methods of serial subculture in growth conditions which promote theselective survival or selective expansion of the definitive endodermcells.

Using the methods described herein, enriched, isolated and/or purifiedpopulations of definitive endoderm cells and or tissues can be producedin vitro from pluripotent cell cultures or cell populations, such asstem cell cultures or populations, which have undergone at least somedifferentiation. In some methods, the cells undergo randomdifferentiation. In a preferred method, however, the cells are directedto differentiate primarily into definitive endoderm. Some preferredenrichment, isolation and/or purification methods relate to the in vitroproduction of definitive endoderm from human embryonic stem cells. Usingthe methods described herein, cell populations or cell cultures can beenriched in definitive endoderm content by at least about 2- to about1000-fold as compared to untreated cell populations or cell cultures.

Compositions Comprising Definitive Endoderm Cells

Cell compositions produced by the above-described methods include cellcultures comprising definitive endoderm and cell populations enriched indefinitive endoderm. For example, cell cultures which comprisedefinitive endoderm cells, wherein at least about 50-99% of the cells inthe cell culture or the cell population are definitive endoderm cells,can be produced. Because the efficiency of the differentiation processcan be adjusted by modifying certain parameters, which include but arenot limited to, cell growth conditions, growth factor concentrations andthe timing of culture steps, the differentiation procedures describedherein can result in about 5%, about 10%, about 15%, about 20%, about25%, about 30%, about 35%, about 40%, about 45%, about 50%, about 55%,about 60%, about 65%, about 70%, about 75%, about 80%, about 85%, about90%, about 95%, about 98%, about 99% or greater than about 99%conversion of pluripotent cells to definitive endoderm. In processes inwhich isolation of definitive endoderm cells is employed, for example,by using an affinity reagent that binds to the CXCR4 receptor, asubstantially pure definitive endoderm cell population can be recovered.In embodiments where the cell cultures or cell populations comprisehuman feeder cells, the above percentages are calculated without respectto the human feeder cells in the cell cultures or cell populations.

Identification of Factors Capable of Promoting, the Differentiation ofPreprimitive Streak and/or Mesendoderm

Certain screening methods described herein relate to methods foridentifying at least one differentiation factor that is capable ofpromoting the differentiation of preprimitive streak and/or mesendodermcells. In some embodiments of these methods, a cell populationcomprising preprimitive streak and/or mesendoderm cells, such as humanpreprimitive streak and/or mesendoderm cells, is obtained. The cellpopulation is then provided with a candidate differentiation factor. Ata first time point, which is prior to or at approximately the same timeas providing the candidate differentiation factor, expression of amarker is determined. Alternatively, expression of the marker can bedetermined after providing the candidate differentiation factor. At asecond time point, which is subsequent to the first time point andsubsequent to the step of providing the candidate differentiation factorto the cell population, expression of the same marker is againdetermined. Whether the candidate differentiation factor is capable ofpromoting the differentiation of the definitive endoderm cells isdetermined by comparing expression of the marker at the first time pointwith the expression of the marker at the second time point. Ifexpression of the marker at the second time point is increased ordecreased as compared to expression of the marker at the first timepoint, then the candidate differentiation factor is capable of promotingthe differentiation of definitive endoderm cells.

Some embodiments of the screening methods described herein utilize cellpopulations or cell cultures which comprise human preprimitive streakand/or mesendoderm cells. For example, the cell population can be asubstantially purified population of human preprimitive streak and/ormesendoderm cells. Alternatively, the cell population can be an enrichedpopulation of human preprimitive streak and/or mesendoderm cells,wherein at least about 90%, at least about 91%, at least about 92%, atleast about 93%, at least about 94%, at least about 95%, at least about96%, at least about 97%, at least about 98%, at least about 99% orgreater than at least about 99% of the human cells in the cellpopulation are human preprimitive streak and/or mesendoderm cells. Inother embodiments described herein, the cell population comprises humancells wherein at least about 10%, at least about 15%, at least about20%, at least about 25%, at least about 30%, at least about 35%, atleast about 40%, at least about 45%, at least about 50%, at least about55%, at least about 60%, at least about 65%, at least about 70%, atleast about 75%, at least about 80%, at least about 85% or greater thanat least about 85% of the human cells are human preprimitive streakand/or mesendoderm cells. In some embodiments, the cell populationincludes non-human cells such as non-human feeder cells. In otherembodiments, the cell population includes human feeder cells. In suchembodiments, at least about 10%, at least about 15%, at least about 20%,at least about 25%, at least about 30%, at least about 35%, at leastabout 40%, at least about 45%, at least about 50%, at least about 55%,at least about 60%, at least about 65%, at least about 70%, at leastabout 75%, at least about 80%, at least about 85%, at least about 90%,at least about 95% or greater than at least about 95% of the humancells, other than said feeder cells, are human preprimitive streakand/or mesendoderm cells.

In embodiments of the screening methods described herein, the cellpopulation is contacted or otherwise provided with a candidate (test)differentiation factor. The candidate differentiation factor cancomprise any molecule that may have the potential to promote thedifferentiation of human preprimitive streak and/or mesendoderm cells.In some embodiments described herein, the candidate differentiationfactor comprises a molecule that is known to be a differentiation factorfor one or more types of cells. In alternate embodiments, the candidatedifferentiation factor comprises a molecule that in not known to promotecell differentiation. In preferred embodiments, the candidatedifferentiation factor comprises molecule that is not known to promotethe differentiation of human preprimitive streak and/or mesendodermcells.

In some embodiments of the screening methods described herein, thecandidate differentiation factor comprises a small molecule. Inpreferred embodiments, a small molecule is a molecule having a molecularmass of about 10,000 amu or less. In some embodiments the small moleculeis a retinoid, such as retinoic acid.

In other embodiments described herein, the candidate differentiationfactor comprises a polypeptide. The polypeptide can be any polypeptideincluding, but not limited to, a glycoprotein, a lipoprotein, anextracellular matrix protein, a cytokine, a chemokine, a peptidehormone, an interleukin or a growth factor. Preferred polypeptidesinclude growth factors. In some preferred embodiments, the candidatedifferentiation factors comprises one or more growth factors selectedfrom the group consisting of FGF10, FGF4, FGF2 and Wnt3B.

In some embodiments of the screening methods described herein, thecandidate differentiation factors comprise one or more growth factorsselected from the group consisting of Amphiregulin, B-lymphocytestimulator, IL-16, Thymopoietin, TRAIL/Apo-2, Pre B cell colonyenhancing factor, Endothelial differentiation-related factor 1 (EDF1),Endothelial monocyte activating polypeptide II, Macrophage migrationinhibitory factor (MIF), Natural killer cell enhancing factor (NKEFA),Bone mophogenetic protein 2, Bone mophogenetic protein 8 (osteogeneicprotein 2), Bone morphogenic protein 6, Bone morphogenic protein 7,Connective tissue growth factor (CTGF), CGI-149 protein (neuroendocrinedifferentiation factor), Cytokine A3 (macrophage inflammatory protein1-alpha), Gliablastoma cell differentiation-related protein (GBDR1),Hepatoma-derived growth factor, Neuromedin U-25 precursor, Vascularendothelial growth factor (VEGF), Vascular endothelial growth factor B(VEGF-B), T-cell specific RANTES precursor, thymic dendriticcell-derived factor 1, Transferrin, Interleukin-1 (IL 1), Interleukin-2(IL 2), Interleukin-3 (IL 3), Interleukin-4 (IL 4), Interleukin-5 (IL5), Interleukin-6 (IL 6), Interleukin-7 (IL 7), Interleukin-8 (IL 8),Interleukin-9 (IL 9), Interleukin-10 (IL 10), Interleukin-11 (IL 11),Interleukin-12 (IL 12), Interleukin-13 (IL 13), Granulocyte-colonystimulating factor (G-CSF), Granulocyte macrophage colony stimulatingfactor (GM-CSF), Macrophage colony stimulating factor (M-CSF),Erythropoietin, Thrombopoietin, Vitamin D₃, Epidermal growth factor(EGF), Brain-derived neurotrophic factor, Leukemia inhibitory factor,Thyroid hormone, Basic fibroblast growth factor (bFGF), aFGF, FGF-4,FGF-6, Keratinocyte growth factor (KGF), Platelet-derived growth factor(PDGF), Platelet-derived growth factor-BB, beta nerve growth factor,activin A, Transforming growth factor beta 1 (TGF-β1), Interferon-α,Interferon-β, Interferon-γ, Tumor necrosis factor-α, Tumor necrosisfactor-β, Burst promoting activity (BPA), Erythroid promoting activity(EPA), PGE₂, insulin growth factor-1 (IGF-1), IGF-II, Neutrophin growthfactor (NGF), Neutrophin-3, Neutrophin 4/5, Ciliary neurotrophic factor,Glial-derived nexin, Dexamethasone, β-mercaptoethanol, Retinoic acid,Butylated hydroxyanisole, 5-azacytidine, Amphotericin B, Ascorbic acid,Ascrorbate, isobutylxanthine, indomethacin, β-glycerolphosphate,nicotinamide, DMSO, Thiazolidinediones, TWS119, oxytocin, vasopressin,melanocyte-stimulating hormone, corticortropin, lipotropin, thyrotropin,growth hormone, prolactin, luteinizing hormone, human chorionicgonadotropin, follicle stimulating hormone, corticotropin-releasingfactor, gonadotropin-releasing factor, prolactin-releasing factor,prolactin-inhibiting factor, growth-hormone releasing factor,somatostatin, thyrotropin-releasing factor, calcitonin gene-relatedpeptide, parathyroid hormone, glucagon-like peptide 1, glucose-dependentinsulinotropic polypeptide, gastrin, secretin, cholecystokinin, motilin,vasoactive intestinal peptide, substance P, pancreatic polypeptide,peptide tyrosine tyrosine, neuropeptide tyrosine, insulin, glucagon,placental lactogen, relaxin, angiotensin II, calctriol, atrialnatriuretic peptide, and melatonin. thyroxine, triiodothyronine,calcitonin, estradiol, estrone, progesterone, testosterone, cortisol,corticosterone, aldosterone, epinephrine, norepinepherine, androstiene,calcitriol, collagen, Dexamethasone, β-mercaptoethanol, Retinoic acid,Butylated hydroxyanisole, 5-azacytidine, Amphotericin B, Ascorbic acid,Ascrorbate, isobutylxanthine, indomethacin, β-glycerolphosphate,nicotinamide, DMSO, Thiazolidinediones, and TWS119.

In some embodiments of the screening methods described herein, thecandidate differentiation factor is provided to the cell population inone or more concentrations. In some embodiments, the candidatedifferentiation factor is provided to the cell population so that theconcentration of the candidate differentiation factor in the mediumsurrounding the cells ranges from about 0.1 ng/ml to about 10 mg/ml. Insome embodiments, the concentration of the candidate differentiationfactor in the medium surrounding the cells ranges from about 1 ng/ml toabout 1 mg/ml. In other embodiments, the concentration of the candidatedifferentiation factor in the medium surrounding the cells ranges fromabout 10 ng/ml to about 100 μg/ml. In still other embodiments, theconcentration of the candidate differentiation factor in the mediumsurrounding the cells ranges from about 100 ng/ml to about 10 μg/ml. Inpreferred embodiments, the concentration of the candidatedifferentiation factor in the medium surrounding the cells is about 5ng/ml, about 25 ng/ml, about 50 ng/ml, about 75 ng/ml, about 100 ng/ml,about 125 ng/ml, about 150 ng/ml, about 175 ng/ml, about 200 ng/ml,about 225 ng/ml, about 250 ng/ml, about 275 ng/ml, about 300 ng/ml,about 325 ng/ml, about 350 ng/ml, about 375 ng/ml, about 400 ng/ml,about 425 ng/ml, about 450 ng/ml, about 475 ng/ml, about 500 ng/ml,about 525 ng/ml, about 550 ng/ml, about 575 ng/ml, about 600 ng/ml,about 625 ng/ml, about 650 ng/ml, about 675 ng/ml, about 700 ng/ml,about 725 ng/ml, about 750 ng/ml, about 775 ng/ml, about 800 ng/ml,about 825 ng/ml, about 850 ng/ml, about 875 ng/ml, about 900 ng/ml,about 925 ng/ml, about 950 ng/ml, about 975 ng/ml, about 1 μg/ml, about2 μg/ml, about 3 μg/ml, about 4 μg/ml, about 5 μg/ml, about 6 μg/ml,about 7 μg/ml, about 8 μg/ml, about 9 μg/ml, about 10 μg/ml, about 11μg/ml, about 12 μg/ml, about 13 μg/ml, about 14 μg/ml, about 15 μg/ml,about 16 μg/ml, about 17 μg/ml, about 18 μg/ml, about 19 μg/ml, about 20μg/ml, about 25 μg/ml, about 50 μg/ml, about 75 μg/ml, about 100 μg/ml,about 125 μg/ml, about 150 μg/ml, about 175 μg/ml, about 200 μg/ml,about 250 μg/ml, about 300 μg/ml, about 350 μg/ml, about 400 μg/ml,about 450 μg/ml, about 500 μg/ml, about 550 μg/ml, about 600 μg/ml,about 650 μg/ml, about 700 μg/ml, about 750 μg/ml, about 800 μg/ml,about 850 μg/ml, about 900 μg/ml, about 950 μg/ml, about 1000 μg/ml orgreater than about 1000 μg/ml.

In certain embodiments of the screening methods described herein, thecell population is provided with a candidate differentiation factorwhich comprises any molecule other than foregut differentiation factor.For example, in some embodiments, the cell population is provided with acandidate differentiation factor which comprises any molecule other thana retinoid, a member of the TGFβ superfamily of growth factors, FGF10 orFGF4. In some embodiments, the cell population is provided with acandidate differentiation factor which comprises any molecule other thanretinoic acid.

In some embodiments, steps of the screening methods described hereincomprise determining expression of at least one marker at a first timepoint and a second time point. In some of these embodiments, the firsttime point can be prior to or at approximately the same time asproviding the cell population with the candidate differentiation factor.Alternatively, in some embodiments, the first time point is subsequentto providing the cell population with the candidate differentiationfactor. In some embodiments, expression of a plurality of markers isdetermined at a first time point.

In addition to determining expression of at least one marker at a firsttime point, some embodiments of the screening methods described hereincontemplate determining expression of at least one marker at a secondtime point, which is subsequent to the first time point and which issubsequent to providing the cell population with the candidatedifferentiation factor. In such embodiments, expression of the samemarker is determined at both the first and second time points. In someembodiments, expression of a plurality of markers is determined at boththe first and second time points. In such embodiments, expression of thesame plurality of markers is determined at both the first and secondtime points. In some embodiments, marker expression is determined at aplurality of time points, each of which is subsequent to the first timepoint, and each of which is subsequent to providing the cell populationwith the candidate differentiation factor. In certain embodiments,marker expression is determined by Q-PCR. In other embodiments, markerexpression is determined by immunocytochemistry.

In certain embodiments of the screening methods described herein, themarker having its expression is determined at the first and second timepoints is a marker that is associated with the differentiation of humanpreprimitive streak cells and/or mesendoderm cells to cells which arethe precursors of cells which make up tissues and/or organs that arederived from the gut tube. In some embodiments, the tissues and/ororgans that are derived from the gut tube comprise terminallydifferentiated cells. In some embodiments, the marker is indicative ofpancreatic cells or pancreatic precursor cells. In preferredembodiments, the marker is pancreatic-duodenal homeobox factor-1 (PDX1).In other embodiments, the marker is homeobox A13 (HOXA13) or homeobox C6(HOXC6). Additionally, in other embodiments, the marker is indicative ofliver cells or liver precursor cells. In certain preferred embodiments,the marker is albumin, hepatocyte specific antigen (HSA) orprospero-related homeobox 1 (PROX1). In other embodiments, the marker isindicative of lung or lung precursor cells. In some preferredembodiments, the marker is thyroid transcription factor 1 (TITF1). Inyet other embodiments, the marker is indicative of intestinal orintestinal precursor cells. In additional preferred embodiments, themarker is villin or caudal type homeobox transcription factor 2 (CDX2).In still other embodiments, the marker is indicative of stomach orstomach precursor cells. In additional preferred embodiments, the markeris VCAM1, VWF or CXCR4. In other embodiments, the marker is indicativeof thyroid or thyroid precursor cells. In such embodiments, the markeris TITF1. In still other embodiments, the marker is indicative of thymusor thymus precursor cells.

In some embodiments of the screening methods described herein,sufficient time is allowed to pass between providing the cell populationwith the candidate differentiation factor and determining markerexpression at the second time point. Sufficient time between providingthe cell population with the candidate differentiation factor anddetermining expression of the marker at the second time point can be aslittle as from about 1 hour to as much as about 10 days. In someembodiments, the expression of at least one marker is determinedmultiple times subsequent to providing the cell population with thecandidate differentiation factor. In some embodiments, sufficient timeis at least about 1 hour, at least about 6 hours, at least about 12hours, at least about 18 hours, at least about 24 hours, at least about30 hours, at least about 36 hours, at least about 42 hours, at leastabout 48 hours, at least about 54 hours, at least about 60 hours, atleast about 66 hours, at least about 72 hours, at least about 78 hours,at least about 84 hours, at least about 90 hours, at least about 96hours, at least about 102 hours, at least about 108 hours, at leastabout 114 hours, at least about 120 hours, at least about 126 hours, atleast about 132 hours, at least about 138 hours, at least about 144hours, at least about 150 hours, at least about 156 hours, at leastabout 162 hours, at least about 168 hours, at least about 174 hours, atleast about 180 hours, at least about 186 hours, at least about 192hours, at least about 198 hours, at least about 204 hours, at leastabout 210 hours, at least about 216 hours, at least about 222 hours, atleast about 228 hours, at least about 234 hours or at least about 240hours.

In some embodiments of the methods described herein, it is furtherdetermined whether the expression of the marker at the second time pointhas increased or decreased as compared to the expression of this markerat the first time point. An increase or decrease in the expression ofthe at least one marker indicates that the candidate differentiationfactor is capable of promoting the differentiation of the definitiveendoderm cells. Similarly, if expression of a plurality of markers isdetermined, it is further determined whether the expression of theplurality of markers at the second time point has increased or decreasedas compared to the expression of this plurality of markers at the firsttime point. An increase or decrease in marker expression can bedetermined by measuring or otherwise evaluating the amount, level oractivity of the marker in the cell population at the first and secondtime points. Such determination can be relative to other markers, forexample housekeeping gene expression, or absolute. In certainembodiments, wherein marker expression is increased at the second timepoint as compared with the first time point, the amount of increase isat least about 2-fold, at least about 5-fold, at least about 10-fold, atleast about 20-fold, at least about 30-fold, at least about 40-fold, atleast about 50-fold, at least about 60-fold, at least about 70-fold, atleast about 80-fold, at least about 90-fold, at least about 100-fold ormore than at least about 100-fold. In some embodiments, the amount ofincrease is less than 2-fold. In embodiments where marker expression isdecreased at the second time point as compared with the first timepoint, the amount of decrease is at least about 2-fold, at least about5-fold, at least about 10-fold, at least about 20-fold, at least about30-fold, at least about 40-fold, at least about 50-fold, at least about60-fold, at least about 70-fold, at least about 80-fold, at least about90-fold, at least about 100-fold or more than at least about 100-fold.In some embodiments, the amount of decrease is less than 2-fold.

In some embodiments of the screening methods described herein, afterproviding the cell population with a candidate differentiation factor,the human preprimitive streak and/or mesendoderm cells differentiateinto one or more cell types of the definitive endoderm lineage. In someembodiments, after providing the cell population with a candidatedifferentiation factor, the human preprimitive streak and/or mesendodermcells differentiate into cells that are derived from the gut tube. Suchcells include, but are not limited to, cells of the pancreas, liver,lungs, stomach, intestine, thyroid, thymus, pharynx, gallbladder andurinary bladder as well as precursors of such cells. Additionally, thesecells can further develop into higher order structures such as tissuesand/or organs.

In other embodiments of the screening methods described herein, afterproviding the cell population with a candidate differentiation factor,the human preprimitive streak and/or mesendoderm cells differentiateinto one or more cell types of the mesoderm lineage. In someembodiments, after providing the cell population with a candidatedifferentiation factor, the human preprimitive streak and/or mesendodermcells differentiate into cells which include, but are not limited to,blood cells, cells of the cardiovascular system, skeletal tissues andother structural and connective tissues as well as precursors of each ofthe aforementioned cell types. Additionally, these cells can furtherdevelop into higher order structures such as tissues and/or organs.

Methods of Increasing the Expression of the FGF8 Gene Product

Some embodiments of the methods described herein relate to a method ofincreasing the expression of the FGF8 gene product in a human embryonicstem cell in vitro. Such methods comprise the step of obtaining saidhESC in a medium comprising less than about 2% (v/v) serum. For example,the medium can comprise serum at a concentration of about 0% (v/v), ofabout 0.05% (v/v), about 0.1% (v/v), about 0.2% (v/v), about 0.3% (v/v),about 0.4% (v/v), about 0.5% (v/v), about 0.6% (v/v), about 0.1% (v/v),about 0.2% (v/v), about 0.3% (v/v), about 0.4% (v/v), about 0.5% (v/v),about 0.6% (v/v), about 0.7% (v/v), about 0.8% (v/v), about 0.9% (v/v),about 1% (v/v), about 1.1% (v/v), about 1.2% (v/v), about 1.3% (v/v),about 1.4% (v/v), about 1.5% (v/v), about 1.6% (v/v), about 1.7% (v/v),about 1.8% (v/v) or about 1.9% (v/v). In some embodiments, the mediumdoes not comprise serum replacement. The hESCs are contacted with adifferentiation factor in an amount sufficient to increase expression ofthe FGF8 gene product. In some embodiments, the differentiation factoris at least one growth factor of the TGFβ superfamily. In preferredembodiments, the growth factor of the TGFβ superfamily is activin A. Theconcentration of differentiation factor that is used to contact thehESCs ranges from about 1 ng/ml to about 1 mg/ml. For example, the hESCscan be contacted with a differentiation factor at a concentration of 1ng/ml, about 5 ng/ml, about 25 ng/ml, about 50 ng/ml, about 75 ng/ml,about 100 ng/ml, about 125 ng/ml, about 150 ng/ml, about 175 ng/ml,about 200 ng/ml, about 225 ng/ml, about 250 ng/ml, about 275 ng/ml,about 300 ng/ml, about 325 ng/ml, about 350 ng/ml, about 375 ng/ml,about 400 ng/ml, about 425 ng/ml, about 450 ng/ml, about 475 ng/ml,about 500 ng/ml, about 525 ng/ml, about 550 ng/ml, about 575 ng/ml,about 600 ng/ml, about 625 ng/ml, about 650 ng/ml, about 675 ng/ml,about 700 ng/ml, about 725 ng/ml, about 750 ng/ml, about 775 ng/ml,about 800 ng/ml, about 825 ng/ml, about 850 ng/ml, about 875 ng/ml,about 900 ng/ml, about 925 ng/ml, about 950 ng/ml, about 975 ng/ml,about 1 μg/ml, about 2 μg/ml, about 3 μg/ml, about 4 μg/ml, about 5μg/ml, about 6 μg/ml, about 7 μg/ml, about 8 μg/ml, about 9 μg/ml, about10 μg/ml, about 11 μg/ml, about 12 μg/ml, about 13 μg/ml, about 14μg/ml, about 15 μg/ml, about 16 μg/ml, about 17 μg/ml, about 18 μg/ml,about 19 μg/ml, about 20 μg/ml, about 25 μg/ml, about 50 μg/ml, about 75μg/ml, about 100 μg/ml, about 125 μg/ml, about 150 μg/ml, about 175μg/ml, about 200 μg/ml, about 250 μg/ml, about 300 μg/ml, about 350μg/ml, about 400 μg/ml, about 450 μg/ml, about 500 μg/ml, about 550μg/ml, about 600 μg/ml, about 650 μg/ml, about 700 μg/ml, about 750μg/ml, about 800 μg/ml, about 850 μg/ml, about 900 μg/ml, about 950μg/ml, about 1000 μg/ml or greater than about 1000 μg/ml.

Methods of Increasing the Expression of the Brachyury, FGF4 and/or SNAI1Gene Product

Other embodiments of the methods described herein relate to a method ofincreasing the expression of the brachyury, FGF4 and/or SNAI1 geneproduct in a human embryonic stem cell in vitro. Such methods comprisethe step of obtaining said hESC in a medium comprising less than about2% (v/v) serum. For example, the medium can comprise serum at aconcentration of about 0% (v/v), of about 0.05% (v/v), about 0.1% (v/v),about 0.2% (v/v), about 0.3% (v/v), about 0.4% (v/v), about 0.5% (v/v),about 0.6% (v/v), about 0.1% (v/v), about 0.2% (v/v), about 0.3% (v/v),about 0.4% (v/v), about 0.5% (v/v), about 0.6% (v/v), about 0.7% (v/v),about 0.8% (v/v), about 0.9% (v/v), about 1% (v/v), about 1.1% (v/v),about 1.2% (v/v), about 1.3% (v/v), about 1.4% (v/v), about 1.5% (v/v),about 1.6% (v/v), about 1.7% (v/v), about 1.8% (v/v) or about 1.9%(v/v). In some embodiments, the medium does not comprise serumreplacement. The hESCs are contacted with a differentiation factor in anamount sufficient to increase expression of the brachyury, FGF4 and/orSNAI1 gene product. In some embodiments, the differentiation factor isat least one growth factor of the TGFβ superfamily. In preferredembodiments, the growth factor of the TGFβ superfamily is activin A. Theconcentration of differentiation factor that is used to contact thehESCs ranges from about 1 ng/ml to about 1 mg/ml. For example, the hESCscan be contacted with a differentiation factor at a concentration of 1ng/ml, about 5 ng/ml, about 25 ng/ml, about 50 ng/ml, about 75 ng/ml,about 100 ng/ml, about 125 ng/ml, about 150 ng/ml, about 175 ng/ml,about 200 ng/ml, about 225 ng/ml, about 250 ng/ml, about 275 ng/ml,about 300 ng/ml, about 325 ng/ml, about 350 ng/ml, about 375 ng/ml,about 400 ng/ml, about 425 ng/ml, about 450 ng/ml, about 475 ng/ml,about 500 ng/ml, about 525 ng/ml, about 550 ng/ml, about 575 ng/ml,about 600 ng/ml, about 625 ng/ml, about 650 ng/ml, about 675 ng/ml,about 700 ng/ml, about 725 ng/ml, about 750 ng/ml, about 775 ng/ml,about 800 ng/ml, about 825 ng/ml, about 850 ng/ml, about 875 ng/ml,about 900 ng/ml, about 925 ng/ml, about 950 ng/ml, about 975 ng/ml,about 1 μg/ml, about 2 μg/ml, about 3 μg/ml, about 4 μg/ml, about 5μg/ml, about 6 μg/ml, about 7 μg/ml, about 8 μg/ml, about 9 μg/ml, about10 μg/ml, about 11 μg/ml, about 12 μg/ml, about 13 μg/ml, about 14μg/ml, about 15 μg/ml, about 16 μg/ml, about 17 μg/ml, about 18 μg/ml,about 19 μg/ml, about 20 μg/ml, about 25 μg/ml, about 50 μg/ml, about 75μg/ml, about 100 μg/ml, about 125 μg/ml, about 150 μg/ml, about 175μg/ml, about 200 μg/ml, about 250 μg/ml, about 300 μg/ml, about 350μg/ml, about 400 μg/ml, about 450 μg/ml, about 500 μg/ml, about 550μg/ml, about 600 μg/ml, about 650 μg/ml, about 700 μg/ml, about 750μg/ml, about 800 μg/ml, about 850 μg/ml, about 900 μg/ml, about 950μg/ml, about 1000 μg/ml or greater than about 1000 μg/ml.

Temporal Expression of Gene Products in Cell Cultures

Further embodiments of the present invention relate to cell cultureshaving certain temporal patterns of gene expression. In someembodiments, the cell culture comprise human embryonic stem cells(hESCs) and a medium comprising less than about 2% (v/v) serum. Forexample, the medium can comprise serum at a concentration of about 0%(v/v), of about 0.05% (v/v), about 0.1% (v/v), about 0.2% (v/v), about0.3% (v/v), about 0.4% (v/v), about 0.5% (v/v), about 0.6% (v/v), about0.1% (v/v), about 0.2% (v/v), about 0.3% (v/v), about 0.4% (v/v), about0.5% (v/v), about 0.6% (v/v), about 0.7% (v/v), about 0.8% (v/v), about0.9% (v/v), about 1% (v/v), about 1.1% (v/v), about 1.2% (v/v), about1.3% (v/v), about 1.4% (v/v), about 1.5% (v/v), about 1.6% (v/v), about1.7% (v/v), about 1.8% (v/v) or about 1.9% (v/v). In some embodiments,the medium does not comprise serum replacement. In some embodiments, themedium is low serum RPMI.

In some embodiments described herein, hESCs in culture begindifferentiating at a reference time point. The reference time point isthe point at which a differentiation factor is provided to the cells. Insome embodiments, the differentiation factor is at least one growthfactor of the TGFβ superfamily. In preferred embodiments, the growthfactor of the TGFβ superfamily is activin A. The concentration ofdifferentiation factor that is provided to the hESCs ranges from about 1ng/ml to about 1 mg/ml. For example, the hESCs can be contacted with adifferentiation factor at a concentration of 1 ng/ml, about 5 ng/ml,about 25 ng/ml, about 50 ng/ml, about 75 ng/ml, about 100 ng/ml, about125 ng/ml, about 150 ng/ml, about 175 ng/ml, about 200 ng/ml, about 225ng/ml, about 250 ng/ml, about 275 ng/ml, about 300 ng/ml, about 325ng/ml, about 350 ng/ml, about 375 ng/ml, about 400 ng/ml, about 425ng/ml, about 450 ng/ml, about 475 ng/ml, about 500 ng/ml, about 525ng/ml, about 550 ng/ml, about 575 ng/ml, about 600 ng/ml, about 625ng/ml, about 650 ng/ml, about 675 ng/ml, about 700 ng/ml, about 725ng/ml, about 750 ng/ml, about 775 ng/ml, about 800 ng/ml, about 825ng/ml, about 850 ng/ml, about 875 ng/ml, about 900 ng/ml, about 925ng/ml, about 950 ng/ml, about 975 ng/ml, about 1 μg/ml, about 2 μg/ml,about 3 μg/ml, about 4 μg/ml, about 5 μg/ml, about 6 μg/ml, about 7μg/ml, about 8 μg/ml, about 9 μg/ml, about 10 μg/ml, about 11 μg/ml,about 12 μg/ml, about 13 μg/ml, about 14 μg/ml, about 15 μg/ml, about 16μg/ml, about 17 μg/ml, about 18 μg/ml, about 19 μg/ml, about 20 μg/ml,about 25 μg/ml, about 50 μg/ml, about 75 μg/ml, about 100 μg/ml, about125 μg/ml, about 150 μg/ml, about 175 μg/ml, about 200 μg/ml, about 250μg/ml, about 300 μg/ml, about 350 μg/ml, about 400 μg/ml, about 450μg/ml, about 500 μg/ml, about 550 μg/ml, about 600 μg/ml, about 650μg/ml, about 700 μg/ml, about 750 μg/ml, about 800 μg/ml, about 850μg/ml, about 900 μg/ml, about 950 μg/ml, about 1000 μg/ml or greaterthan about 1000 μg/ml.

After providing the differentiation factor to the hESCs, the expressionof FGF8 mRNA is substantially upregulated as compared to baseline FGF8mRNA expression in the hESCs. Baseline FGF8 expression in hESCs is theexpression of FGF8 gene product, such as mRNA, present in an hESC cellculture that is maintained in it undifferentiated state. In someembodiments, FGF8 mRNA expression is substantially upregulated by about6 hours from the reference time point. In further embodiments describedherein, expression of FGF8 mRNA is downregulated after about 24 hoursfrom the reference time point. In other embodiments, peak expression ofFGF8 mRNA is reached at a time between about 6 hours and about 24 hoursfrom the reference time point. In other embodiments, peak expression ofFGF8 may be reached at a time less than about 6 hours from the referencetime point.

Other embodiments described herein relate to cell cultures exhibitingincreased nuclear localization of the β-catenin polypeptide. In suchembodiments, the β-catenin polypeptide begins to become localized to thecell nucleus by about 17 hours from the reference time point. In someembodiments, the β-catenin polypeptide begins to become localized to thecell nucleus by less than about 17 hours from the reference time point.In still other embodiments, the β-catenin polypeptide becomespredominantly localized to the cell nucleus by about 17 hours from thereference time point.

Still other embodiments of the cell cultures described herein relate tocell cultures having increased expression of brachyury mRNA. In suchcell cultures, brachyury mRNA expression is substantially upregulated byabout 24 hours from the reference time point. In some embodiments,brachyury mRNA expression is substantially upregulated prior to about 24hours from the reference time point. In some embodiments, expression ofbrachyury mRNA is substantially downregulated by about 48 hours from thereference time point. In certain embodiments, peak expression ofbrachyury mRNA is reached at a time between about 12 hours and about 48hours from the reference time point. In preferred embodiments, thebrachyury mRNA in not substantially expressed by about 72 hours from thereference time point. In other preferred embodiments, brachyury mRNA issubstantially upregulated by about 24 hours from the reference timepoint and is not substantially expressed by about 72 hours from thereference time point.

Still further embodiments of the cell cultures described herein relateto cell cultures having increased expression of FGF4 mRNA. In such cellcultures, FGF4 mRNA expression is substantially upregulated by about 24hours from the reference time point. In some embodiments, FGF4 mRNAexpression is substantially upregulated prior to about 24 hours from thereference time point. In some embodiments, expression of FGF4 mRNA issubstantially downregulated by about 48 hours from the reference timepoint. In certain embodiments, peak expression of FGF4 mRNA is reachedat a time between about 12 hours and about 48 hours from the referencetime point. In preferred embodiments, the FGF4 mRNA in not substantiallyexpressed by about 72 hours from the reference time point. In otherpreferred embodiments, FGF4 mRNA is substantially upregulated by about24 hours from the reference time point and is not substantiallyexpressed by about 72 hours from the reference time point.

Preferred embodiments of the cell cultures described herein relate tocell cultures having increased expression of brachyury and FGF4 mRNA. Insuch cell cultures, brachyury and FGF4 mRNA expression is substantiallyupregulated by about 24 hours from the reference time point. In someembodiments, brachyury and FGF4 mRNA expression is substantiallyupregulated prior to about 24 hours from the reference time point. Insome embodiments, expression of brachyury and FGF4 mRNA is substantiallydownregulated by about 48 hours from the reference time point. Incertain embodiments, peak expression of brachyury and FGF4 mRNA isreached at a time between about 12 hours and about 48 hours from thereference time point. In preferred embodiments, the brachyury and FGF4mRNA in not substantially expressed by about 72 hours from the referencetime point. In other preferred embodiments, brachyury and FGF4 mRNA issubstantially upregulated by about 24 hours from the reference timepoint and is not substantially expressed by about 72 hours from thereference time point.

Additional embodiments of the cell cultures described herein relate tocell cultures having increased expression of SNAI1 mRNA. In such cellcultures, SNAI1 mRNA expression is substantially upregulated by about 24hours from the reference time point. In some embodiments, SNAI1 mRNAexpression is substantially upregulated prior to about 24 hours from thereference time point. In some embodiments, expression of SNAI1 mRNA isdownregulated by about 48 hours from the reference time point. Incertain embodiments, peak expression of SNAI1 mRNA is reached at a timebetween about 12 hours and about 48 hours from the reference time point.

Embodiments of the cell cultures described herein also relate to cellcultures having a specific temporal expression of the E-cadherin (ECAD)gene product. In such embodiments, expression of E-cadherin mRNA beginsto be downregulated by about 12 hours from the reference time point. Inother embodiments, expression of E-cadherin mRNA can be downregulated byabout less than 12 hours from the reference time point. In preferredembodiments, expression of E-cadherin mRNA is substantiallydownregulated by about 48 hours from the reference time point.

Further embodiments described herein relate to cell cultures thatexpress the SOX17 and/or FOXA2 marker. In some embodiments, expressionof SOX17 mRNA is substantially upregulated by about 48 hours from thereference time point. In other embodiments, expression of FOXA2 mRNA issubstantially upregulated by about 96 hours from the reference timepoint.

Some embodiments described herein relate to cell cultures that comprisecells having certain specified patterns of gene expression. In suchembodiments, the cell cultures comprise hESCs, a differentiation factor,such as a differentiation factor of the TGFβ superfamily, and a mediumcomprising less than about 2% (v/v) serum. In alternative embodiments,the medium comprises greater than 2% (v/v) serum. In some embodiments,the medium lacks serum replacement.

In some embodiments, at the time the cell culture is provided with thedifferentiation factor, the cell culture comprises all or predominantlyall hESCs. During the course of differentiation, at least a portion ofthe hESCs differentiate into other cell types as indicated by theexpression of the products (mRNA and/or polypeptides) of certain markergenes.

In some embodiments of the cell cultures described herein, theexpression of a first set of marker genes is upregulated prior to theupregulation of a second and/or a third set of marker genes. In someembodiments, each set of marker genes can include one or more markergenes. Upregulation of gene expression can range from slight tosubstantial. For example, expression of a marker gene can be upregulatedby at least about 10% as compared to the expression of the same markergene in undifferentiated hESCs. In other embodiments, expression of amarker gene can be upregulated by at least about 20%, at least about30%, at least about 40%, at least about 50%, at least about 60%, atleast about 70%, at least about 80%, at least about 90% or greater thanat least about 90% as compared to the expression of the same marker genein undifferentiated hESCs. In still other embodiments, the marker geneexpression can be upregulated by at least 2-fold, at least about 3-fold,at least about 4-fold, at least about 5-fold, at least about 6-fold, atleast about 7-fold, at least about 8-fold, at least about 9-fold, atleast about 10-fold, at least about 15-fold, at least about 20-fold, atleast about 30-fold, at least about 40-fold, at least about 50-fold, atleast about 60-fold, at least about 70-fold, at least about 80-fold, atleast about 90-fold, at least about 100-fold or greater than at leastabout 100-fold as compared to the expression of the same marker gene inundifferentiated hESCs.

In other embodiments of the cell cultures described herein, theexpression of a first set of marker genes is downregulated regulatedprior to the upregulation of a second and/or a third set of markergenes. In such embodiments, the each set of marker genes can include oneor more marker genes. As with upregulation of gene expression,downregulation can range from slight to substantial. For example,expression of a marker gene can be downregulated by at least about 10%as compared to the expression of the same marker gene inundifferentiated hESCs. In other embodiments, expression of a markergene can be downregulated by at least about 20%, at least about 30%, atleast about 40%, at least about 50%, at least about 60%, at least about70%, at least about 80%, at least about 90% or greater than at leastabout 90% as compared to the expression of the same marker gene inundifferentiated hESCs. In still other embodiments, the marker geneexpression can be downregulated by at least 2-fold, at least about3-fold, at least about 4-fold, at least about 5-fold, at least about6-fold, at least about 7-fold, at least about 8-fold, at least about9-fold, at least about 10-fold, at least about 15-fold, at least about20-fold, at least about 30-fold, at least about 40-fold, at least about50-fold, at least about 60-fold, at least about 70-fold, at least about80-fold, at least about 90-fold, at least about 100-fold or greater thanat least about 100-fold as compared to the expression of the same markergene in undifferentiated hESCs.

In still other embodiments of the cell cultures described herein, theexpression of a first set of marker genes is upregulated prior to or atabout the same time as the peak expression of a second set and/or athird set of marker genes. In such embodiments, each set of marker genescan comprise one or more marker genes. In other embodiments, theexpression of a first set of marker genes is downregulated prior to orat about the same time as the peak expression of a second set and/or athird set of marker genes. As described above, in such embodiments, eachset of marker genes can comprise one or more marker genes. Furthermore,in the above-described embodiments, both upregulation and downregulationof gene expression can range from slight to substantial. For example,expression of a marker gene can be upregulated or downregulated by atleast about 10% as compared to the expression of the same marker gene inundifferentiated hESCs. In other embodiments, expression of a markergene can be upregulated or downregulated by at least about 20%, at leastabout 30%, at least about 40%, at least about 50%, at least about 60%,at least about 70%, at least about 80%, at least about 90% or greaterthan at least about 90% as compared to the expression of the same markergene in undifferentiated hESCs. In still other embodiments, the markergene expression can be upregulated or downregulated by at least 2-fold,at least about 3-fold, at least about 4-fold, at least about 5-fold, atleast about 6-fold, at least about 7-fold, at least about 8-fold, atleast about 9-fold, at least about 10-fold, at least about 15-fold, atleast about 20-fold, at least about 30-fold, at least about 40-fold, atleast about 50-fold, at least about 60-fold, at least about 70-fold, atleast about 80-fold, at least about 90-fold, at least about 100-fold orgreater than at least about 100-fold as compared to the expression ofthe same marker gene in undifferentiated hESCs.

In some embodiments described herein, in at least some cells of the cellculture, expression of a marker gene selected from the group consistingof FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4. In other embodiments, in at leastsome cell of the cell culture, expression of a marker gene selected fromthe group consisting of FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 isupregulated prior to peak expression of a marker gene selected from thegroup consisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4. In still other embodiments, in atleast some cell of the cell culture, expression of a marker geneselected from the group consisting of HEY1, GATA2, BIK and ID1 isdownregulated prior to or at about the same time as peak expression of amarker gene selected from the group consisting of brachyury, FGF4,SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662, SLIT2, GAD1 andGRM4. In further embodiments, in at least some cell of the cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of SOX17, FOXA2, CXCR4 and MIXL1. In still other embodiments,in at least some cell of the cell culture, expression of a marker geneselected from the group consisting of brachyury, FGF4, SNAI1, Wnt3,MIXL1, DKK4, NETO1, T, DACT1, FLJ22662, SLIT2, GAD1 and GRM4 isupregulated prior to or at about the same time as upregulation ofexpression of a marker gene selected from the group consisting of SOX17,FOXA2, CXCR4 and MIXL1. In yet other embodiments, in at least some cellof the cell culture, peak expression of a marker gene selected from thegroup consisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4 is reached prior to or at aboutthe same time as upregulation of expression of a marker gene selectedfrom the group consisting of SOX17, FOXA2, CXCR4 and MIXL1. Inadditional embodiments, in at least some cell of the cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

Some of the cell cultures having one or more of the temporal geneexpression patterns described above include a medium comprising lessthan about 2% (v/v) serum. In some embodiments, the medium does notinclude serum. In other embodiments, the medium lacks serum replacement.Some of the media used in the cell cultures described herein includeserum at a concentration of less than about 1.9% (v/v), less than about1.8% (v/v), less than about 1.7% (v/v), less than about 1.6% (v/v), lessthan about 1.5% (v/v), less than about 1.4% (v/v), less than about 1.3%(v/v), less than about 1.2% (v/v), less than about 1.1% (v/v), less thanabout 1% (v/v), less than about 0.9% (v/v), less than about 0.8% (v/v),less than about 0.7% (v/v), less than about 0.6% (v/v), less than about0.5% (v/v), less than about 0.4% (v/v), less than about 0.3% (v/v), lessthan about 0.2% (v/v), less than about 0.1% (v/v) or less than about0.05% (v/v).

Some of the cell cultures having one or more of the temporal geneexpression patterns described above comprise at least onedifferentiation factor of the TGFβ superfamily. In some embodiments, thegrowth factor is nodal, activin A and/or activin B. In preferredembodiments, the differentiation factor is activin A. In more preferredembodiments, the activin A is present in the medium at a concentrationof about 100 ng/ml.

It will be appreciated, however, that the differentiation factor of theTGFβ superfamily can be supplied to the cell culture at concentrationsranging from about 1 ng/ml to about 1 mg/ml. In some embodiments, thedifferentiation factor of the TGFβ superfamily is supplied to the cellculture at about 5 ng/ml, about 25 ng/ml, about 50 ng/ml, about 75ng/ml, about 100 ng/ml, about 125 ng/ml, about 150 ng/ml, about 175ng/ml, about 200 ng/ml, about 225 ng/ml, about 250 ng/ml, about 275ng/ml, about 300 ng/ml, about 325 ng/ml, about 350 ng/ml, about 375ng/ml, about 400 ng/ml, about 425 ng/ml, about 450 ng/ml, about 475ng/ml, about 500 ng/ml, about 525 ng/ml, about 550 ng/ml, about 575ng/ml, about 600 ng/ml, about 625 ng/ml, about 650 ng/ml, about 675ng/ml, about 700 ng/ml, about 725 ng/ml, about 750 ng/ml, about 775ng/ml, about 800 ng/ml, about 825 ng/ml, about 850 ng/ml, about 875ng/ml, about 900 ng/ml, about 925 ng/ml, about 950 ng/ml, about 975ng/ml, about 1 μg/ml, about 2 μg/ml, about 3 μg/ml, about 4 μg/ml, about5 μg/ml, about 6 μg/ml, about 7 μg/ml, about 8 μg/ml, about 9 μg/ml,about 10 μg/ml, about 11 μg/ml, about 12 μg/ml, about 13 μg/ml, about 14μg/ml, about 15 μg/ml, about 16 μg/ml, about 17 μg/ml, about 18 μg/ml,about 19 μg/ml, about 20 μg/ml, about 25 μg/ml, about 50 μg/ml, about 75μg/ml, about 100 μg/ml, about 125 μg/ml, about 150 μg/ml, about 175μg/ml, about 200 μg/ml, about 250 μg/ml, about 300 μg/ml, about 350μg/ml, about 400 μg/ml, about 450 μg/ml, about 500 μg/ml, about 550μg/ml, about 600 μg/ml, about 650 μg/ml, about 700 μg/ml, about 750μg/ml, about 800 μg/ml, about 850 μg/ml, about 900 μg/ml, about 950μg/ml, about 1000 μg/ml or greater than about 1000 μg/ml.

Methods of differentiating hESCs so as to produce cells having certaintemporal marker gene expression patterns are also contemplated herein.For example, some embodiments relate to a method of differentiatinghuman embryonic stem cells (hESCs), by contacting the hESCs with amedium comprising less that about 2% serum, providing the hESCs with adifferentiation factor of the TGFβ superfamily, and then permittingdifferentiation of the hESCs to occur.

In some embodiments of the above-described methods of differentiatinghESCs, the expression of a first set of marker genes is upregulatedprior to the upregulation of a second and/or a third set of markergenes. In some embodiments, each set of marker genes can include one ormore marker genes. Upregulation of gene expression can range from slightto substantial. For example, expression of a marker gene can beupregulated by at least about 10% as compared to the expression of thesame marker gene in undifferentiated hESCs. In other embodiments,expression of a marker gene can be upregulated by at least about 20%, atleast about 30%, at least about 40%, at least about 50%, at least about60%, at least about 70%, at least about 80%, at least about 90% orgreater than at least about 90% as compared to the expression of thesame marker gene in undifferentiated hESCs. In still other embodiments,the marker gene expression can be upregulated by at least 2-fold, atleast about 3-fold, at least about 4-fold, at least about 5-fold, atleast about 6-fold, at least about 7-fold, at least about 8-fold, atleast about 9-fold, at least about 10-fold, at least about 15-fold, atleast about 20-fold, at least about 30-fold, at least about 40-fold, atleast about 50-fold, at least about 60-fold, at least about 70-fold, atleast about 80-fold, at least about 90-fold, at least about 100-fold orgreater than at least about 100-fold as compared to the expression ofthe same marker gene in undifferentiated hESCs.

In other embodiments of methods of differentiating hESCs describedherein, the expression of a first set of marker genes is downregulatedregulated prior to the upregulation of a second and/or a third set ofmarker genes. In such embodiments, the each set of marker genes caninclude one or more marker genes. As with upregulation of geneexpression, downregulation can range from slight to substantial. Forexample, expression of a marker gene can be downregulated by at leastabout 10% as compared to the expression of the same marker gene inundifferentiated hESCs. In other embodiments, expression of a markergene can be downregulated by at least about 20%, at least about 30%, atleast about 40%, at least about 50%, at least about 60%, at least about70%, at least about 80%, at least about 90% or greater than at leastabout 90% as compared to the expression of the same marker gene inundifferentiated hESCs. In still other embodiments, the marker geneexpression can be downregulated by at least 2-fold, at least about3-fold, at least about 4-fold, at least about 5-fold, at least about6-fold, at least about 7-fold, at least about 8-fold, at least about9-fold, at least about 10-fold, at least about 15-fold, at least about20-fold, at least about 30-fold, at least about 40-fold, at least about50-fold, at least about 60-fold, at least about 70-fold, at least about80-fold, at least about 90-fold, at least about 100-fold or greater thanat least about 100-fold as compared to the expression of the same markergene in undifferentiated hESCs.

In still other embodiments of methods of differentiating hESCs describedherein, the expression of a first set of marker genes is upregulatedprior to or at about the same time as the peak expression of a secondset and/or a third set of marker genes. In such embodiments, each set ofmarker genes can comprise one or more marker genes. In otherembodiments, the expression of a first set of marker genes isdownregulated prior to or at about the same time as the peak expressionof a second set and/or a third set of marker genes. As described above,in such embodiments, each set of marker genes can comprise one or moremarker genes. Furthermore, in the above-described embodiments, bothupregulation and downregulation of gene expression can range from slightto substantial. For example, expression of a marker gene can beupregulated or downregulated by at least about 10% as compared to theexpression of the same marker gene in undifferentiated hESCs. In otherembodiments, expression of a marker gene can be upregulated ordownregulated by at least about 20%, at least about 30%, at least about40%, at least about 50%, at least about 60%, at least about 70%, atleast about 80%, at least about 90% or greater than at least about 90%as compared to the expression of the same marker gene inundifferentiated hESCs. In still other embodiments, the marker geneexpression can be upregulated or downregulated by at least 2-fold, atleast about 3-fold, at least about 4-fold, at least about 5-fold, atleast about 6-fold, at least about 7-fold, at least about 8-fold, atleast about 9-fold, at least about 10-fold, at least about 15-fold, atleast about 20-fold, at least about 30-fold, at least about 40-fold, atleast about 50-fold, at least about 60-fold, at least about 70-fold, atleast about 80-fold, at least about 90-fold, at least about 100-fold orgreater than at least about 100-fold as compared to the expression ofthe same marker gene in undifferentiated hESCs.

In some embodiments described herein, in at least some cells of the cellculture, expression of a marker gene selected from the group consistingof FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4. In other embodiments, in at leastsome cell of the cell culture, expression of a marker gene selected fromthe group consisting of FGF8, Nodal, HEG, HEY1, GATA2, BIK and ID1 isupregulated prior to peak expression of a marker gene selected from thegroup consisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4. In still other embodiments, in atleast some cell of the cell culture, expression of a marker geneselected from the group consisting of HEY1, GATA2, BIK and ID1 isdownregulated prior to or at about the same time as peak expression of amarker gene selected from the group consisting of brachyury, FGF4,SNAI1, Wnt3, MIXL1, DKK4, NETO1, T, DACT1, FLJ22662, SLIT2, GAD1 andGRM4. In further embodiments, in at least some cell of the cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of SOX17, FOXA2, CXCR4 and MIXL1. In still other embodiments,in at least some cell of the cell culture, expression of a marker geneselected from the group consisting of brachyury, FGF4, SNAI1, Wnt3,MIXL1, DKK4, NETO1, T, DACT1, FLJ22662, SLIT2, GAD1 and GRM4 isupregulated prior to or at about the same time as upregulation ofexpression of a marker gene selected from the group consisting of SOX17,FOXA2, CXCR4 and MIXL1. In yet other embodiments, in at least some cellof the cell culture, peak expression of a marker gene selected from thegroup consisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4 is reached prior to or at aboutthe same time as upregulation of expression of a marker gene selectedfrom the group consisting of SOX17, FOXA2, CXCR4 and MIXL1. Inadditional embodiments, in at least some cell of the cell culture,expression of a marker gene selected from the group consisting of FGF8,Nodal, HEG, HEY1, GATA2, BIK and ID1 is upregulated prior toupregulation of expression of a marker gene selected from the groupconsisting of brachyury, FGF4, SNAI1, Wnt3, MIXL1, DKK4, NETO1, T,DACT1, FLJ22662, SLIT2, GAD1 and GRM4.

In some embodiments of the differentiation methods described herein,cells of the cell culture are contacted or otherwise provided with amedium that comprises less than about 2% (v/v) serum. In some embodimentthe medium does not include serum. In other embodiments, the mediumlacks serum replacement. Some of the media used in the methods describedherein include serum at a concentration of less than about 1.9% (v/v),less than about 1.8% (v/v), less than about 1.7% (v/v), less than about1.6% (v/v), less than about 1.5% (v/v), less than about 1.4% (v/v), lessthan about 1.3% (v/v), less than about 1.2% (v/v), less than about 1.1%(v/v), less than about 1% (v/v), less than about 0.9% (v/v), less thanabout 0.8% (v/v), less than about 0.7% (v/v), less than about 0.6%(v/v), less than about 0.5% (v/v), less than about 0.4% (v/v), less thanabout 0.3% (v/v), less than about 0.2% (v/v), less than about 0.1% (v/v)or less than about 0.05% (v/v).

Some of the methods for differentiating hESCs to produce theabove-described temporal patterns of gene expression include providingthe cells in culture, such as hESCs, with at least one differentiationfactor of the TGFβ superfamily. In some embodiments, the growth factoris nodal, activin A and/or activin B. In preferred embodiments, thedifferentiation factor is activin A. In more preferred embodiments, theactivin A is present in the medium at a concentration of about 100ng/ml.

It will be appreciated, however, that the differentiation factor of theTGFβ superfamily can be supplied to the cell culture at concentrationsranging from about 1 ng/ml to about 1 mg/ml. In some embodiments, thedifferentiation factor of the TGFβ superfamily is supplied to the cellculture at about 5 ng/ml, about 25 ng/ml, about 50 ng/ml, about 75ng/ml, about 100 ng/ml, about 125 ng/ml, about 150 ng/ml, about 175ng/ml, about 200 ng/ml, about 225 ng/ml, about 250 ng/ml, about 275ng/ml, about 300 ng/ml, about 325 ng/ml, about 350 ng/ml, about 375ng/ml, about 400 ng/ml, about 425 ng/ml, about 450 ng/ml, about 475ng/ml, about 500 ng/ml, about 525 ng/ml, about 550 ng/ml, about 575ng/ml, about 600 ng/ml, about 625 ng/ml, about 650 ng/ml, about 675ng/ml, about 700 ng/ml, about 725 ng/ml, about 750 ng/ml, about 775ng/ml, about 800 ng/ml, about 825 ng/ml, about 850 ng/ml, about 875ng/ml, about 900 ng/ml, about 925 ng/ml, about 950 ng/ml, about 975ng/ml, about 1 μg/ml, about 2 μg/ml, about 3 μg/ml, about 4 μg/ml, about5 μg/ml, about 6 μg/ml, about 7 μg/ml, about 8 μg/ml, about 9 μg/ml,about 10 μg/ml, about 11 μg/ml, about 12 μg/ml, about 13 μg/ml, about 14μg/ml, about 15 μg/ml, about 16 μg/ml, about 17 μg/ml, about 18 μg/ml,about 19 μg/ml, about 20 μg/ml, about 25 μg/ml, about 50 μg/ml, about 75μg/ml, about 100 μg/ml, about 125 μg/ml, about 150 μg/ml, about 175μg/ml, about 200 μg/ml, about 250 μg/ml, about 300 μg/ml, about 350μg/ml, about 400 μg/ml, about 450 μg/ml, about 500 μg/ml, about 550μg/ml, about 600 μg/ml, about 650 μg/ml, about 700 μg/ml, about 750μg/ml, about 800 μg/ml, about 850 μg/ml, about 900 μg/ml, about 950μg/ml, about 1000 μg/ml or greater than about 1000 μg/ml.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only, and are not intended to belimiting.

EXAMPLES

Many of the examples below describe the use of pluripotent human cells.Methods of producing pluripotent human cells are well known in the artand have been described numerous scientific publications, including U.S.Pat. Nos. 5,453,357, 5,670,372, 5,690,926, 6,090,622, 6,200,806 and6,251,671 as well as U.S. Patent Application Publication No.2004/0229350, the disclosures of which are incorporated herein byreference in their entireties.

Example 1 Human ES Cells

For our studies of early development we employed human embryonic stemcells, which are pluripotent and can divide seemingly indefinitely inculture while maintaining a normal karyotype. ES cells were derived fromthe 5-day-old embryo inner cell mass using either immunological ormechanical methods for isolation. In particular, the human embryonicstem cell line hESCyt-25 was derived from a supernumerary frozen embryofrom an in vitro fertilization cycle following informed consent by thepatient. Upon thawing the hatched blastocyst was plated on mouseembryonic fibroblasts (MEF), in ES medium (DMEM, 20% FBS, non essentialamino acids, beta-mercaptoethanol, ITS supplement). The embryo adheredto the culture dish and after approximately two weeks, regions ofundifferentiated hESCs were transferred to new dishes with MEFs.Transfer was accomplished with mechanical cutting and a brief digestionwith dispase, followed by mechanical removal of the cell clusters,washing and re-plating. Since derivation, hESCyt-25 has been seriallypassaged over 100 times. We employed the hESCyt-25 human embryonic stemcell line as our starting material for the production of definitiveendoderm.

It will be appreciated by those of skill in the art that stem cells orother pluripotent cells can also be used as starting material for thedifferentiation procedures described herein. For example, cells obtainedfrom embryonic gonadal ridges, which can be isolated by methods known inthe art, can be used as pluripotent cellular starting material.

Example 2 hESCyt-25 Characterization

The human embryonic stem cell line, hESCyt-25 has maintained a normalmorphology, karyotype, growth and self-renewal properties over 18 monthsin culture. This cell line displays strong immunoreactivity for theOCT4, SSEA-4 and TRA-1-60 antigens, all of which, are characteristic ofundifferentiated hESCs and displays alkaline phosphatase activity aswell as a morphology identical to other established hESC lines.Furthermore, the human stem cell line, hESCyt-25, also readily formsembryoid bodies (EBs) when cultured in suspension. As a demonstration ofits pluripotent nature, hESCyT-25 differentiates into various cell typesthat represent the three principal germ layers. Ectoderm production wasdemonstrated by Q-PCR for ZIC1 as well as immunocytochemistry (ICC) fornestin and more mature neuronal markers. Immunocytochemical staining forβ-III tubulin was observed in clusters of elongated cells,characteristic of early neurons. Previously, we treated EBs insuspension with retinoic acid, to induce differentiation of pluripotentstem cells to visceral endoderm (VE), an extra-embryonic lineage.Treated cells expressed high levels of α-fetoprotein (AFP) and SOX7, twomarkers of VE, by 54 hours of treatment. Cells differentiated inmonolayer expressed AFP in sporadic patches as demonstrated byimmunocytochemical staining. As will be described below, the hESCyT-25cell line was also capable of forming definitive endoderm, as validatedby real-time quantitative polymerase chain reaction (Q-PCR) andimmunocytochemistry for SOX17, in the absence of AFP expression. Todemonstrate differentiation to mesoderm, differentiating EBs wereanalyzed for Brachyury gene expression at several time points. Brachyuryexpression increased progressively over the course of the experiment. Inview of the foregoing, the hESCyT-25 line is pluripotent as shown by theability to form cells representing the three germ layers.

Example 3 Definitive Endoderm Cells

Copending and co-owned U.S. patent application Ser. No. 11/021,618,entitled DEFINITIVE ENDODERM, filed Dec. 23, 2004, describes cellcultures and enriched cell populations comprising human definitiveendoderm cells. Also described therein are methods of producingdefinitive endoderm from hESCs by differentiation in the presence of adifferentiation factor as well as methods for enriching, isolatingand/or purifying these definitive endoderm cells from mixed cellcultures and/or cell populations. Further described therein are markersthat are useful for the identification and/or detection of definitiveendoderm cells as well as a marker useful in the purification of suchcells. The disclosed of U.S. patent application Ser. No. 11/021,618,entitled DEFINITIVE ENDODERM, filed Dec. 23, 2004, is incorporatedherein by reference in its entirety.

This Example, which describes useful marker for the detection and/oridentification of definitive endoderm cells, it reproduced from theaforementioned copending patent application.

In the following experiment, RNA was isolated from purified definitiveendoderm and human embryonic stem cell populations. Gene expression wasthen analyzed by gene chip analysis of the RNA from each purifiedpopulation. Q-PCR was also performed to further investigate thepotential of genes expressed in definitive endoderm, but not inembryonic stem cells, as a marker for definitive endoderm.

Human embryonic stem cells (hESCs) were maintained in DMEM/F12 mediasupplemented with 20% KnockOut Serum Replacement, 4 ng/ml recombinanthuman basic fibroblast growth factor (bFGF), 0.1 mM 2-mercaptoethanol,L-glutamine, non-essential amino acids and penicillin/streptomycin.hESCs were differentiated to definitive endoderm by culturing for 5 daysin RPMI media supplemented with 100 ng/ml of recombinant human activinA, fetal bovine serum (FBS), and penicillin/streptomycin. Theconcentration of FBS was varied each day as follows: 0.1% (first day),0.2% (second day), 2% (days 3-5).

Cells were isolated by fluorescence activated cell sorting (FACS) inorder to obtain purified populations of hESCs and definitive endodermfor gene expression analysis. Immuno-purification was achieved for hESCsusing SSEA4 antigen (R&D Systems, cat# FAB1435P) and for definitiveendoderm using CXCR4 (R&D Systems, cat# FAB170P). Cells were dissociatedusing trypsin/EDTA (Invitrogen, cat#25300-054), washed in phosphatebuffered saline (PBS) containing 2% human serum and resuspended in 100%human serum on ice for 10 minutes to block non-specific binding.Staining was carried out for 30 minutes on ice by adding 200 μl ofphycoerythrin-conjugated antibody to 5×10⁶ cells in 800 μl human serum.Cells were washed twice with 8 ml of PBS buffer and resuspended in 1 mlof the same, FACS isolation was carried out by the core facility of TheScripps Research Institute using a FACS Vantage (BD Biosciences). Cellswere collected directly into RLT lysis buffer and RNA was isolated byRNeasy according to the manufacturers instructions (Qiagen).

Purified RNA was submitted in duplicate to Expression Analysis (Durham,N.C.) for generation of the expression profile data using the Affymetrixplatform and U133 Plus 2.0 high-density oligonucleotide arrays. Datapresented is a group comparison that identifies genes differentiallyexpressed between the two populations, hESCs and definitive endoderm.Genes that exhibited a robust upward change in expression level overthat found in hESCs were selected as new candidate markers that arehighly characteristic of definitive endoderm. Select genes were assayedby Q-PCR, as described above, to verify the gene expression changesfound on the gene chip and also to investigate the expression pattern ofthese genes during a time course of hESC differentiation.

FIGS. 2A-M show the gene expression results for certain markers. Resultsare displayed for cell cultures analyzed 1, 3 and 5 days after theaddition of 100 ng/ml activin A, CXCR4-expressing definitive endodermcells purified at the end of the five day differentiation procedure(CXDE), and in purified hESCs. A comparison of FIGS. 2C and G-Mdemonstrates that the six marker genes, FGF17, VWF, CALCR, FOXQ1, CMKOR1and CRIP1, exhibited an expression pattern that is almost identical toeach other and which is also identical to the pattern of expression ofCXCR4 and the ratio of SOX17/SOX7. As described previously, SOX17 wasexpressed in both the definitive endoderm as well as in theSOX7-expressing extra-embryonic endoderm. Since SOX7 was not expressedin the definitive endoderm, the ratio of SOX17/SOX7 provides a reliableestimate of definitive endoderm contribution to the SOX17 expressionwitnessed in the population as a whole. The similarity of panels G-L andM to panel C indicates that FGF17, VWF, CALCR, FOXQ1, CMKOR1 and CRIP1are markers of definitive endoderm and that they are not significantlyexpressed in extra-embryonic endoderm cells.

It will be appreciated that the Q-PCR results described herein can befurther confirmed by ICC.

Example 4 Temporal Sequence of Gene Expression in Definitive EndodermPrecursor Cells

To assess the dynamics of gene expression that occur duringdifferentiation to definitive endoderm, the expression of numerous geneswas monitored in differentiating cell cultures at various time pointsduring two different four day differentiation protocols.

Specifically, hESCs were cultured in the absence of FBS for the first 24hours, in 0.2% (v/v) FBS for the second 24 hours and in 2% (v/v) FBS ondays 3 and 4, with continuous exposure to either 100 ng/ml of activin Ato induce differentiation to definitive endoderm or 100 ng/ml of BMP4and 2.5 μM of the FGFR1 inhibitor, SU5402, to induce differentiation toa mixed cell population of trophectoderm (TE) and primitive endoderm(PrE). The dynamics of gene expression in such cultures were determinedby measuring gene expression at various time points during thedifferentiation procedure. In particular, gene expression was determinedby Q-PCR at the time of addition of the differentiation factors as wellas 6 hours, 12 hours, 24 hours, 48 hours, 72 hours and 96 hours afterthe addition of these factors. Gene expression profiles over this timecourse for several different marker genes are displayed in FIGS. 3A-L.

FIG. 3A shows that expression of FGF8 increased to near its maximumlevel within 6 hours after treatment with activin A. FGF8 expressionremained high for about 24 hours and then began to decline thereafter.Treatment with BMP4/SU5402 caused very little change in the level ofFGF8 expression. (FIG. 3A). These results demonstrate that activin Amediates a rapid transition of hESCs to FGF8 expressing cells. As FGF8expression is one the first indicators of posterior pattern formation inthe epiblast, the rapid up-regulation of FGF8 in differentiating hESCsindicates that these early stage cells have differentiated away from thehESC cell type to form a “pre-streak” (preprimitive streak) cellpopulation.

As shown in FIGS. 3B-F, the induction of certain primitive streak(mesendoderm) markers, such as brachyury, FGF4, SNAI1, MIXL1 and GSC,began at about 6 hours after treatment with activin A. However, incontrast to FGF8 induction, expression of these markers continued toincrease and reached maximal levels at about 24 hours, or in some cases48 hours, after the addition of activin A. Little to no increasedexpression of any of the primitive streak markers was observed in theBMP4/SU5402 treated cultures (FIGS. 3B-F). Furthermore, the expressionlevels of brachyury and FGF4, which are not expressed in definitiveendoderm cells, down-regulated extremely rapidly to at or below hESClevels by 48-72 hours post activin A treatment (FIGS. 3B-C). Incontrast, SNAI1, MIXL1, and GSC continued to be significantly expressedat 48-96 hours (FIGS. 3D-F).

At about 72 hours after treatment with activin A, SOX17 displayed robustup-regulation, which is consistent with a transition from a primitivestreak (mesendoderm) intermediate to a definitive endoderm cell fate(FIG. 3G). No such upregulation of SOX17 expression occurred in theBMP4/SU5402 treated cell cultures (FIG. 3G).

This temporal sequence of gene expression just described for the activinA-treated, in vitro cell cultures mimics events that occur in earlyvertebrate gastrulation, and thus, suggests that the hESCs aretransitioning through similar intermediates during this differentiationprocess.

In addition to the foregoing sequence of events leading to definitiveendoderm, FIGS. 3H-L show that at about 24 to 48 hours, robustupregulation of trophectoderm and primitive endoderm markers, such asHCG, DAB2, SOX7, CHRD and E-cadherin (ECAD), began in cells treated withBMP4/SU5402. The upregulation of these markers continued through 72 and96 hours. In contrast, no upregulation of HCG, DAB2, SOX7, CHRD orE-cadherin occurred in the cell cultures exposed to 100 ng/ml activin A(FIGS. 3H-L). These results indicate that hESCs have the potential todifferentiate to trophectoderm and primitive endoderm under appropriateconditions, however, differentiation to these lineages did not occur toa significant degree in activin treated cultures.

Example 5 Human Embryonic Stem Cell Epithelial to Mesenchymal Transition

During vertebrate gastrulation epiblast cells undergoing epithelial tomesenchymal transition (EMT) at the primitive streak alter theexpression pattern of E-cadherin protein on the cell surface. Thecombined actions of FGF and TGFβ signaling serve to induce thezinc-finger transcription factor SNAI1, which is a directtranscriptional repressor of E-cadherin. This Example shows thattranscription of E-cadherin is repressed during the earlydifferentiation stages of activin-exposed hESC cultures in vitro just asin the EMT that occurs in vivo during gastrulation.

Human embryonic stem cell cultures were differentiated for two days inthe presence of 100 ng/ml activin A in RMPI medium containing 0.1% (v/v)FBS. Expression of certain marker genes was determined using Q-PCRand/or immunocytochemistry. FIGS. 4A-C show the Q-PCR-determinedexpression of brachyury, E-cadherin and SNAI1 mRNA, respectively.Immunocytochemistry was used to determine the expression of brachyury,E-cadherin and activated (unphosphorylated) B-catenin.

As described previously, activin exposed preprimitive streak cells inculture differentiated to primitive streak (mesendoderm) cells afterabout 24 hours of activin exposure (Example 4 and FIG. 7). A comparisonof FIG. 4A with 4C shows that both brachyury and SNAI1 mRNA levelspeaked at about 24 hours. A similar peak in expression was seen for FGF4mRNA (FIG. 3C). In relation to brachyury expression during this timeperiod, the expression of E-cadherin mRNA was decreasing and continuedto decrease more than 3-fold by 48 hours after the initiation of activinA treatment (FIG. 4B). By 12 hours post activin exposure, whenE-Cadherin mRNA levels had dropped by about 33% from their originallevels in the differentiating cell culture, there was a simultaneousdisappearance of the characteristic apical cell surfaceimmunolocalization of E-cadherin (presumably involved in adherensjunctions) in brachyury positive cells at the periphery of colonies.After 24 hours, as the number of brachyury expressing cells increased,the loss of junction-associated E-Cadherin in brachyury positive cellswas more robust. In addition, the morphology of the cells appeared lessuniform as evidenced by the differences in morphology of DAPI andbrachyury-stained nuclei at both 12 and 24 hours, further supportingoccurrence of an EMT.

In addition to the foregoing gene expression, the intracellularlocalization of β-catenin was determined as hESCs differentiated todefinitive endoderm. Using an antibody that recognized only theunphosphorylated (Ser37 and Thr41) form of β-catenin, it was found thatthis protein was localized only at the cytoplasmic surface inundifferentiated hESCs. By 17 hours post activin treatment, both nuclearand membrane bound β-catenin was detected in both brachyury-negative andbrachyury-expressing cells. Furthermore, western blot analyses confirmedthe presence of β-catenin in the soluble, membrane-excluded proteinfractions of differentiating but not undifferentiated cells. Theseresults are consistent with the hypothesis that the nuclear localizationof β-catenin occurs in advance of brachyury protein expression.Additionally, taken together, the above data suggest that inactivin-exposed cell cultures, as the primitive streak-like statedevelops, a classical EMT occurs in which activated β-catenintranslocates to the nucleus in advance of the expression of brachyuryprotein and coincides with disassembly of adherens junctions at theepithelial borders between cells.

Example 6 Differentiation of Mesendoderm to Definitive Endoderm

This Example demonstrates the competency of mesendoderm precursor cellsto differentiate into definitive endoderm.

Human embryonic stem cell cultures were incubated for two days in thepresence of 100 ng/ml activin A in RPMI medium having a serumconcentration of 0.1% (v/v) FSB. As shown in FIG. 5A, brachyury mRNAexpression peaked at about 24 hours. At this same time, the expressionof the SOX17 gene began to increase (FIG. 5B). By about 48 hours,brachyury mRNA expression had returned to baseline levels (FIG. 5A) andSOX17 was expressed at about 18-fold above baseline (FIG. 5B). Theseresults, which have been verified more than seven independentexperiments, are consistent with the hypothesis that brachyuryexpressing mesendoderm cells begin a conversion to SOX17-expressingdefinitive endoderm at around 36-48 hours post activin treatment.

To demonstrate the validity of the above hypothesis,immunocytochemistry-based analysis of the brachyury and SOX17 markerswas performed. The time course for brachyury protein expression showsthe appearance of immunoreactive cells which began at the periphery ofmost large colonies at 12 hrs and rapidly spread to the interior of thecolonies by 36-48 hours. At these later times, the majority of the cellsin the culture were positive for brachyury protein. As such, thebrachyury protein expression pattern was delayed by 12-24 hours inrelation to the peak of brachyury mRNA expression shown in FIG. 5A.After 48 hours of differentiation, when SOX17 mRNA and proteinexpression were rapidly increasing and brachyury expression was rapidlydecreasing, most of the SOX17 positive cells co-expressed brachyury(FIGS. 6A-D). These data clearly demonstrate that, in the presence of100 ng/ml activin and low serum, the majority of SOX17 positive cellswere derived from brachyury-positive, mesendoderm cells. Furthermore,this finding is highly suggestive of the existence of a mesendodermintermediate in human development. Additionally, since brachyuryexpression does not occur in cells of the primitive endoderm lineages atany time during development, these findings conclusively prove thedefinitive nature of the activin-produced endoderm.

Example 7 Formation of Mesoderm and Definitive Endoderm from MesendodermCells

In addition to the foregoing Example, experiments were conducted todemonstrate that the mesendoderm intermediates described herein canundergo differentiation to either mesoderm or definitive endoderm cells.

FIG. 7 shows the experimental design and culture conditions used in thisExample. Specifically, two parallel hESCs were cultured for four days inRPMI medium. As in Example 4, the serum concentration of this medium wasadjusted such that it contained 0% (v/v) FBS on the first day, 0.2%(v/v) FBS on the second day and 2% (v/v) FBS on the third and fourthdays. In each of the parallel cultures, activin A was supplied in theculture medium at a concentration of 100 ng/ml for the first 24 hours.At the end of this time period, activin A was removed from one of theparallel cultures (NF) and maintained in the other (A100).

FIGS. 8A-F show that there was a lack of expression for the mesodermmarkers brachyury, MOX1, FOXF1, FLK1, BMP4, and SDF1 under high activinand low FBS conditions. These results demonstrate that thebrachyury/FGF4/MIXL1-expressing mesendoderm cells were patterned towardsthe production of definitive endoderm rather than to mesoderm under highactivin A conditions. This was also demonstrated by the robustexpression of definitive endoderm markers, GSC, SOX17 and FOXA2 in theculture treated with activin over the entire four day period (FIGS.9A-C). In contrast, removal of activin A at 24 hours, which simulateslower nodal/activin signaling, resulted in a dramatic loss of definitiveendoderm gene expression (FIGS. 9A-C) simultaneously with a dramaticgain of mesoderm character (FIGS. 8A-F).

Example 8 Definitive Endoderm Production in Medium Containing 0.5% Serum

To further demonstrate the effect of serum reduction on the productionof definitive endoderm cells, hESCs were differentiated to definitiveendoderm cells medium containing various serum concentrations.

Human embryonic stem cells were cultured for five days in RPMI mediumcontaining 100 ng/ml activin A in the presence of 10% (v/v) FBS, 2%(v/v) FBS or 0.5% (v/v) FBS. Control cell cultures containing no activinA were also established at each of the three test serum levels. For eachof these cultures, expression of several marker genes was determined byQ-PCR (FIGS. 10A-I). Additionally, the amount of the SOX17 proteinproduced by cells in these cultures was determined byimmunocytochemistry using the antibody to SOX17 that has been previouslydescribed herein.

FIGS. 10A-I show the expression of certain definitive endoderm markergenes at varying concentrations of FBS. In particular, FIGS. 10A-D showthat decreasing the serum concentration in cell cultures treated withactivin A resulted in substantial increases in the production of mRNAcorresponding to the definitive endoderm positive markers, SOX17, GSC,MIXL1 and FOXA2. In the case of markers for which mRNA expression wasdecreased in response to activin A (markers of non-definitive endodermcell types, including Brachyury, MOX1, SOX7, SOX1 and ZIC1), decreasingthe serum concentrations resulted in substantial decreases in theproduction of mRNA corresponding to these markers (FIGS. 10E-I).

The relative proportion of hESC converted to definitive endoderm cellsat each of the serum concentrations was determined byimmunocytochemistry using antibody to SOX17. After 5 days ofdifferentiation in 100 ng/ml activin A in 0.5% FBS, the proportion ofSOX17 positive cells was greater than 80% (>3 separate experiments) andthere was no detectable immunoreactivity for alpha-fetoprotein (AFP) orthrombomodulin (THBD), markers of VE/PE and trophectoderm (TE),respectively. The correlation between increasing SOX17 mRNA expressionand increasing number of SOX17 immunoreactive cells across theseconditions indicates that the relative gene expression measurement isalso a reasonable measurement of SOX17-positive cell number within thepopulation.

Example 9 Upregulation of SOX17 Expression Precedes the Upregulation ofFOXA2 Expression

This Example demonstrates that in activin-mediated differentiation ofhESCs to definitive endoderm, SOX17 mRNA expression is upregulated priorto FOXA2 mRNA expression.

Human embryonic stem cells were cultured in RPMI medium for four days inthe presence of activin A and low serum as described previously inExample 4. Expression of SOX17 and FOXA2 mRNA was determined by Q-PCR.

As shown in FIG. 11A, SOX17 mRNA was substantially increased beginningat about 48 hours post activin A addition. A similar substantialincrease in expression for FOXA2 mRNA was not observed until about 96hours post activin A addition. These results are consistent withproduction of definitive endoderm and not axial mesoderm (see also FIGS.9A-C). These results, however, contrast with the temporal expression ofSOX17 and FOXA2 in mouse embryoid bodies incubated with activins whereFOXA2 expression precedes that of SOX17. As such, it is likely that themouse embryoid body systems are optimized for the production of axialmesoderm rather than definitive endoderm.

Example 10 Production of Definitive Endoderm in Eight Different hESCLines

This Example shows that definitive endoderm cells can be produced fromeight independently derived hESC lines using the methods describedherein.

Eight different hESC lines (CyT25, CyT-DM3, BG01, BG02, BG03, H7, H9 andHUES7) were cultured separately for five days in the presence of 100ng/ml activin A in low serum RPMI medium. Specifically, the RPMI mediumcontained 0% (v/v) FBS on day 1, 0.2% (v/v) FBS on day 2 and 2% (v/v)FBS on days 3-5.

Each of the eight cell lines differentiated to SOX17/CXCR4-positivedefinitive endoderm cells.

Example 11 Transplantation of Human Definitive Endoderm Cells UnderMouse Kidney Capsule

To demonstrate that the human definitive endoderm cells produced usingthe methods described herein are capable of responding todifferentiation factors so as to produce cells derived from the guttube, such human definitive endoderm cells were subjected to an in vivodifferentiation protocol.

Human definitive endoderm cells were produced as described in theforegoing Examples. Such cells were harvested and transplanted under thekidney capsule of immunocompromised mice using standard procedures.After three weeks, the mice were sacrificed and the transplanted tissuewas removed, sectioned and subjected to histological andimmunocytochemical analysis.

FIGS. 12A-D show that after three weeks post-transplantation, the humandefinitive endoderm cells differentiated into cells and cellularstructures derived from the gut tube. In particular, FIG. 12A showshematoxylin and eosin stained sections of transplanted human definitiveendoderm tissue that has differentiated into gut-tube-like structures.FIG. 12B shows a transplanted human definitive endoderm sectionimmunostained with antibody to hepatocyte specific antigen (HSA). Thisresult indicates that the human definitive endoderm cells are capable ofdifferentiating into liver or liver precursor cells. FIGS. 12C and 12Dshow a transplanted human definitive endoderm section immunostained withantibody to villin and antibody to caudal type homeobox transcriptionfactor 2 (CDX2), respectively. These results indicate that the humandefinitive endoderm cells are capable of differentiating into intestinalcells or intestinal cell precursors.

Example 12 Generation of FGF8 Promoter-EGFP and Brachyury Promoter-EGFPTransgenic Human Embryonic Stem Cell Lines

In order to use the FGF8 and brachyury markers for cell isolation, hESCshaving FGF8 or brachyury gene promoters fused with an expressiblereporter gene are constructed. In particular, this Example describes theconstruction of a vector comprising a reporter cassette which comprisesa reporter gene under the control of the FGF8 regulatory region.Additionally, the construction of a vector comprising a reportercassette which comprises a reporter gene under the control of thebrachyury regulatory region is described. This Example also describesthe preparation of a cell, such as a human embryonic stem cell,transfected with one or more of these vectors as well as a cell havingthis one or both of these reporter cassettes integrated into its genome.

FGF8-expressing preprimitive streak cell lines and brachyury-expressingmesendoderm cell lines genetically tagged with a reporter gene areconstructed by placing a GFP reporter gene under the control of theregulatory region (promoter) of the FGF8 gene or the brachyury gene,respectively. First, a plasmid construct in which EGFP expression isdriven by the human FGF8 or brachyury gene promoter is generated byreplacing the CMV promoter of vector pEGFP-N1 (Clontech) with the humanFGF8 or brachyury control region. These control regions contain thecharacterized regulatory elements of either the FGF8 or the brachyurygene, and they is sufficient to confer the normal expression pattern ofthese genes in transgenic mice. In the resulting vector, expression ofEFGP is driven by either the FGF8 promoter or the brachyury promoter. Insome experiments, this vector can be transfected into hESCs.

The FGF8 promoter/EGFP cassette or the brachyury promoter/EGFP cassetteis excised from the above vector, and then subcloned into a selectionvector containing the neomycin phosphotransferase gene under control ofthe phosphoglycerate kinase-1 promoter. The selection cassette isflanked by flp recombinase recognition sites to allow removal of thecassette. This selection vector is linearized, and then introduced intohESCs using standard lipofection methods. Following 10-14 days ofselection in G418, undifferentiated transgenic hESC clones are isolatedand expanded.

It will be appreciated that vectors containing GFP or EGFP reportergenes under the control of the FGF4 or SNAI1 promoter can also beconstructed. Furthermore, it will be appreciated that reporter genesother than GFP or EGFP can be used in any of the above-describedconstructs provided that the reporter allows for cell separation byFACS.

Example 13 Production of Cell Populations Enriched in PreprimitiveStreak Cells

The following Example demonstrates that hESCs comprising an FGF8promoter/EGFP cassette can be differentiated into preprimitive streakcells and then subsequently isolated by fluorescence-activated cellsorting (FACS).

FGF8 promoter/EGFP transgenic hESCs are differentiated for approximately6, 12 and 18 hours in growth medium containing 100 ng/ml activin A andno serum. The differentiated cells are then harvested by trypsindigestion and sorted on a Becton Dickinson FACS Diva directly into RNAlysis buffer or PBS. A sample of single live cells is taken withoutgating for EGFP and single live cells are gated into EGFP positive andGFP negative populations. In a separate experiment, the EGFP positivefraction is separated into two equally sized populations according tofluorescence intensity (Hi and Lo).

Following sorting, cell populations are analyzed by both Q-PCR andimmunocytochemistry. For Q-PCR analysis, RNA is prepared using QiagenRNeasy columns and then converted to cDNA. Q-PCR is conducted asdescribed previously. For immunocytochemistry analysis, cells are sortedinto PBS, fixed for 10 minutes in 4% paraformaldehyde, and adhered toglass slides using a Cytospin centrifuge. The primary antibody is toβ-catenin. An appropriate secondary antibody conjugated to FITC (green)or Rhodamine (Red) is used to detect binding of the primary antibody.

Sorted cells are further subjected to Q-PCR analysis. Differentiatedcells show a correlation of EGFP fluorescence with endogenous FGF8 geneexpression. Compared to non-fluorescing cells, the EGFP positive cellsshow a greater than 2-fold increase in FGF8 expression levels. Theseparation of high and low EGFP intensity cells indicates that EGFPexpression level correlates with FGF8 expression level. In addition toFGF8 marker analysis, sorted cells are subjected to immunocytochemistryanalysis of nuclear-localized β-catenin (nuclear β-catenin). Nuclearlocalization of this marker is enriched in the EGFP positive fraction.In contrast, little nuclear localization of β-catenin is seen in theEGFP negative fraction.

Given these results, at least about 5% of the cells present in thedifferentiated cell cultures prior to sorting are FGF8/nuclearβ-catenin-positive cells. At least about 90% of the cells in the sortedcell populations are FGF8/nuclear β-catenin-positive preprimitive streakcells.

Example 14 Production of Cell Populations Enriched in Mesendoderm Cells

The following Example demonstrates that hESCs comprising a brachyurypromoter/EGFP cassette can be differentiated into primitive streak(mesendoderm) cells and then subsequently isolated byfluorescence-activated cell sorting (FACS).

Brachyury promoter/EGFP transgenic hESCs are differentiated forapproximately 24 hours in growth medium containing 100 ng/ml activin Aand no serum or 100 ng/ml activin A and 0.1% (v/v) FBS. Thedifferentiated cells are then harvested by trypsin digestion and sortedon a Becton Dickinson FACS Diva directly into RNA lysis buffer or PBS. Asample of single live cells is taken without gating for EGFP and singlelive cells are gated into EGFP positive and GFP negative populations. Ina separate experiment, the EGFP positive fraction is separated into twoequally sized populations according to fluorescence intensity (Hi andLo).

Following sorting, cell populations are analyzed by both Q-PCR andimmunocytochemistry. For Q-PCR analysis, RNA is prepared using QiagenRNeasy columns and then converted to cDNA. Q-PCR is conducted asdescribed previously. For immunocytochemistry analysis, cells are sortedinto PBS, fixed for 10 minutes in 4% paraformaldehyde, and adhered toglass slides using a Cytospin centrifuge. The primary antibodies are toFGF4 and SNAI1. Appropriate secondary antibodies conjugated to FITC(green) or Rhodamine (Red) are used to detect binding of the primaryantibodies.

Sorted cells are further subjected to Q-PCR analysis. Differentiatedcells show a correlation of EGFP fluorescence with endogenous brachyurygene expression. Compared to non-fluorescing cells, the EGFP positivecells show a greater than 2-fold increase in brachyury expressionlevels. The separation of high and low EGFP intensity cells indicatesthat EGFP expression level correlates with brachyury expression level.In addition to brachyury marker analysis, sorted cells are subjected toimmunocytochemistry analysis of FGF4 and SNAI1. Each of these markersare enriched in the EGFP positive fraction. In contrast, little FGF4and/or SNAI1 is seen in the EGFP negative fraction.

Given these results, at least about 5% of the cells present in thedifferentiated cell cultures prior to sorting arebrachyury/FGF4/SNAI1-positive cells. At least about 90% of the cells inthe sorted cell populations are brachyury/FGF4/SNAI1-positivemesendoderm cells.

It will be appreciated that, as an alternative to vectors comprising anEGFP/brachyury construct, vectors comprising a GFP or EGFP reporter geneunder the control of the FGF4 or SNAI1 promoter can be used in themesendoderm enrichment methods described in this Example.

Example 15 Identification of Differentiation Factors Capable ofPromoting the Differentiation of Human Preprimitive Steak and/orMesendoderm Cells In Vitro

To exemplify the differentiation factor screening methods describedherein, populations of human preprimitive streak and mesendoderm cellsproduced using the methods described herein are separately provided withseveral candidate differentiation factors while determining thenormalized expression levels of certain marker gene products at varioustime points.

Human preprimitive streak and mesendoderm cell cultures are produced asdescribed in the foregoing Examples. In brief, hESCs cells are grown inthe presence of 100 ng/ml activin A in RPMI medium without serum for 12hours (preprimitive streak) or 24 hours (mesendoderm). After formationof preprimitive streak cells and mesendoderm cells, the cell populationsare maintained in individual plates in RPMI containing 0.2% FBS and aretreated with one of: Wnt3B at 20 ng/ml, FGF2 at 5 ng/ml or FGF2 at 100ng/ml. The expression of marker gene products for albumin, PROX1 andTITF1 are quantitated using Q-PCR.

Incubation of the preprimitive streak cells and the mesendoderm cellswith the above-mentioned molecules is expected to cause the preprimitivestreak cells and the mesendoderm cells to differentiate to cells derivedfrom the definitive endoderm lineage.

Example 16 Summary of Marker Expression in Early Embryonic Cell Types

This Example provides a Table which summarizes the expression of markergenes useful in the identification and/or detection of cell typesobtained during the early differentiation of hESCs. In Table 1 below thefull name and abbreviation for each marker is provided in the first twocolumns. Each of the next columns describes whether the gene isexpressed in a specific tissue type. The following abbreviations forcell type are used: ICM/ESC refers to hESCs; PS refers to primitivestreak (mesendoderm) cells; Ecto refers to ectoderm cells; Meso refersto mesoderm cells; DE refers to definitive endoderm cells; PrE refers toprimitive endoderm; VE refers visceral endoderm; and PE refers toparietal endoderm.

TABLE 1 Expression domains - gastrulation stage ICM/ PrE/ Gene Name GeneAbb. ESC PS Ecto Meso DE VE/PE TE fibroblast growth factor 8 FGF8 − +− + + + − brachyury BRACH − + − + − − − fibroblast growth factor 4FGF4 + + − + − − − snail homolog 1 SNAI1 − + − + + + + (Drosophila) Mixlhomeobox (Xenopus MIXL1 − + − + + + − laevis)-like 1 goosecoid GSC − +− + + + − SRY (sex determining SOX17 − − − − + + − region Y)-box 17chemokine (C—X—C motif) CXCR4 − − − + + − − receptor 4 hepatocytenuclear factor 3, FOXA2 − + − + + + − beta disabled homolog 2 DAB2 − − −− − + − SRY (sex determining SOX7 − − − − − + − region Y)-box 7 chordinCHRD − + − + + + − E-cadherin (epithelial) ECAD + − + − +/− + + SRY (sexdetermining SOX1 − − + − − − − region Y)-box 1 zinc finger protein ofthe ZIC1 − − +/− +/− − − − cerebellum 1 POU domain, class 5, OCT4 + − −− − + − transcription factor 1 forkhead box F1 FOXF1 − − − + − − −mesenchyme homeo box 1 MOX1 − − − + − − − VEGFR, kinase insert FLK1 − −− + − − − domain receptor bone morphogenetic protein 4 BMP4 − − − + −− + stromal cell-derived factor 1 SDF1 unk − − + − + + chorionicgonadotropin, HCG − − − − − − + beta polypeptide 5

Example 17 Additional Markers Useful for Identifying and/or DetectingPreprimitive Streak Cells and Mesendoderm Cells

This Example describes additional markers that are useful in identifyingand/or detecting preprimitive streak and/or mesendoderm cells. It willbe appreciated that the preprimitive streak markers described in thisExample can be used in place of or in addition to any of thepreviously-described markers for preprimitive streak cells. It will alsobe appreciated that the mesendoderm markers described in this Examplecan be used in place of or in addition to any of thepreviously-described markers for mesendoderm cells.

In order to investigate the process of hESC differentiation topreprimitive streak, mesendoderm and subsequently to endoderm andmesoderm, we conducted global expression profiling of differentiatinghESC cultures at short time increments under low serum (≦2% (v/v) FBS)and high activin A (100 ng/ml), with or without the addition of the FGFreceptor inhibitor, SU5402, after the first 24 hours of differentiation.Adding SU5402 to the activin A at 24 hours resulted in a drasticdecrease in expression of genes characteristic of definitive endodermand greatly increases the expression of mesoderm markers.

To mediate cell differentiation, hESCs were grown in low serum RPMImedium in the presence of 100 ng/ml activin A for 4 days. Serumsupplementation was 0% (v/v) for the first 24 hours, 0.2% (v/v) for thesecond 24 hours and 2% (v/v) for days 3 and 4 of differentiation. Afterthe first 24 hours of differentiation, mesoderm was induced in one setof cultures by the application of SU5402 at 5 μM in addition to theactivin A. The other set of cultures remained in activin A alone andthus produced definitive endoderm at high efficiency. Samples were takenin duplicate at 0, 2, 6, 24, 30, 48 and 96 hour time points. The 48 and96 hour points were either definitive endoderm (activin A alone) ormesoderm (activin A with SU5402). Total RNA was extracted and submittedto Expression Analysis (Durham, N.C.) for global expression profilingusing the Affymetrix high-density oligonucleotide arrays (U133 plus2.0).

Gene expression patterns over the time course of hESC differentiationwere studied using hierarchical clustering analyses as well ascomparative analyses between groups of paired samples at the varioustime points. Genes that displayed an expression pattern characterized bya robust increase in expression over the first 6 hours followed byeither maintenance or decreases in expression at the later time pointswere identified as genes involved in differentiation of hESCs topreprimitive streak cells. Genes that displayed peaks in expression atthe 24-30 hour time points were identified as genes involved inmesendoderm differentiation.

Table 2 below describes marker genes that were expressed at highlyelevated levels by 6 hours after contacting the hESCs with activin A.These genes are indicative of preprimitive streak cells. Table 3, belowdescribes marker genes that were expressed at peak levels by about 24 toabout 30 hours after contacting the hESCs with activin A. These genesare indicative of mesendoderm cells. In each of these tables, the firstcolumn lists the probe set identification number and the second columnprovides the Unigene Identification Number. The Unigene database ispublicly available via the National Center for Biotechnology Information(NCBI) website. The nucleotide and amino acid sequences for any of themarker genes described in these tables can therefore be obtained by aquery for the appropriate Unigene Identification Number at the NCBIwebsite. The Unigene database is incorporated herein by reference in itsentirety. Column three of the each table provides the commonly knowngene symbol for each of the markers. The remainder of the columnsprovide the relative levels of gene expression at the 0, 2, 6, 24, 30,48 and 96 hour time points. As used in these tables, “DE” refers todefinitive endoderm and “M” refers to mesoderm.

TABLE 2 Probe Set Unigene Gene 48 hr- 96 hr- Id Id Symbol 0 hr 2 hr 6 hr24 hr 30 hr DE 48 hr-M DE 96 hr-M 208449_s_at Hs.57710 FGF8 47 212 534741 513 390 939 701 93 230916_at Hs.370414 NODAL 2799 4147 9451 978912388 6368 8090 8202 1921 213069_at Hs.477420 HEG 76 160 720 567 915 970619 719 481 218839_at Hs.234434 HEY1 30 233 999 202 160 38 98 33 768209710_at Hs.367725 GATA2 75 283 903 317 197 94 345 132 930 205780_atHs.475055 BIK 418 1061 2403 804 367 165 1157 746 1003 208937_s_atHs.504609 ID1 5378 13769 21979 8886 7177 8627 15486 6468 24305

TABLE 3 Probe Set Unigene Gene 48 hr- 96 hr- Id Id Symbol 0 hr 2 hr 6 hr24 hr 30 hr DE 48 hr-M DE 96 hr-M 206783_at Hs.1755 FGF4 1138 1401 15584521 3291 2538 3581 181 932 229103_at Hs.445884 WNT3 234 240 717 1415859 1123 758 881 77 231746_at Hs.282079 MIXL1 286 427 1252 12014 1291111598 4121 5674 152 206619_at Hs.159311 DKK4 35 14 16 3876 3218 3877 719 22 219480_at Hs.48029 SNAI1 78 131 249 525 434 536 338 243 2611562713_a_at Hs.465407 NETO1 104 174 192 330 283 165 71 64 19 206524_atHs.389457 T (brachyury) 581 552 722 1761 1816 604 344 159 123 219179_atHs.48950 DACT1 824 683 702 2734 2023 1016 467 285 680 218454_atHs.131933 FLJ22662 974 1318 1106 3651 2532 1205 1048 874 1294209897_s_at Hs.29802 SLIT2 223 244 212 547 625 321 173 145 285 205278_atHs.420036 GAD1 160 167 212 676 723 292 444 264 18 210234_at Hs.429018GRM4 384 359 389 738 823 216 116 139 213

Example 18 Markers Useful for Identifying and/or Detecting EmbryonicStem Cell Differentiation Prior to the PrePrimitive Streak Stage(Primitive Ectoderm Cells)

This Example describes markers that are useful in identifying and/ordetecting stem cell differentiation prior to the preprimitive streakstage. Such cell types are referred to here as primitive ectoderm cells.It will be appreciated that the primitive ectoderm cell markersdescribed in this Example can be associated with primitive ectoderm cellcompositions and used in the methods that have already been describedfor producing and screening preprimitive streak cells and/or mesendodermcells.

In order to investigate the process of hESC differentiation to primitiveectoderm, we conducted global expression profiling of differentiatinghESC cultures as described in Example 17.

Table 4 below describes marker genes that were expressed at increased ordecreased elevated levels prior to 6 hours after contacting the hESCswith activin A. These genes are indicative of primitive ectoderm cells.As indicated for Tables 2 and 3 above, in Table 4, the first columnlists the probe set identification number and the second column providesthe Unigene Identification Number. The Unigene database is publiclyavailable via the National Center for Biotechnology Information (NCBI)website. The nucleotide and amino acid sequences for any of the markergenes described in this table can therefore be obtained by a query forthe appropriate Unigene Identification Number at the NCBI website. TheUnigene database is incorporated herein by reference in its entirety.Column three of the Table 4 provides the commonly known gene symbol foreach of the markers. The remainder of the columns provide the relativelevels of gene expression at the 0, 2, 6, 24, 30, 48 and 96 hour timepoints. As used in this table, “DE” refers to definitive endoderm and“M” refers to mesoderm.

Table 4 shows that the expression of FZD10, FGF5 and OCT4 mRNAs issubstantially upregulated by 2 hours after contacting the hESCs with 100ng/ml activin A. In contrast, the expression of GBX2, ZFP42 and SOX2mRNA is substantially downregulated by 2 hours after activin Atreatment. The expression of Nanog mRNA is expected to be substantiallyupregulated at a time prior to six hours post activin A treatment.

TABLE 4 Probe Set Unigene Gene 48 hr- 96 hr- Id Id Symbol 0 hr 2 hr 6 hr24 hr 30 hr DE 48 hr-M DE 96 hr-M 219764_at Hs.31664 FZD10 141 788 318314 200 67 134 35 9 210311_at Hs.37055 FGF5 59 130 98 70 49 221 88 16192 220184_at Hs.329926 NANOG 8341 8065 14341 8716 7906 4436 8874 39771291 214532_x_at Hs.249184 OCT4 26278 33882 33953 29083 25912 1573620166 7488 18338 210560_at Hs.184945 GBX2 161 80 46 29 10 56 43 10 131554776_at Hs.335787 ZFP42 2727 1445 2213 1723 1493 1011 1489 526 2329213722_at Hs.518438 SOX2 937 643 518 387 354 193 719 108 472

The methods, compositions, and devices described herein are presentlyrepresentative of preferred embodiments and are exemplary and are notintended as limitations on the scope of the invention. Changes thereinand other uses will occur to those skilled in the art which areencompassed within the spirit of the invention and are defined by thescope of the disclosure. Accordingly, it will be apparent to one skilledin the art that varying substitutions and modifications may be made tothe invention disclosed herein without departing from the scope andspirit of the invention.

As used in the claims below and throughout this disclosure, by thephrase “consisting essentially of” is meant including any elementslisted after the phrase, and limited to other elements that do notinterfere with or contribute to the activity or action specified in thedisclosure for the listed elements. Thus, the phrase “consistingessentially of” indicates that the listed elements are required ormandatory, but that other elements are optional and may or may not bepresent depending upon whether or not they affect the activity or actionof the listed elements.

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What is claimed is:
 1. An in vitro method of producing mesendodermcells, the method comprising the steps of: (a) obtaining a cellpopulation comprising human pluripotent stem cells; (b) providing thehuman pluripotent stem cells with at least one TGFβ superfamily growthfactor in an amount sufficient to promote differentiation of the humanpluripotent stem cells to mesendoderm cells, said mesendoderm cellsbeing multipotent cells that can differentiate into mesoderm ordefinitive endoderm cells; and (c) allowing sufficient time for themesendoderm cells to form, wherein the sufficient time for themesendoderm cells to form has been determined by detecting the presenceof mesendoderm cells in the cell population.
 2. The method of claim 1,wherein, the TGFβ superfamily growth factor is selected from the groupconsisting of Nodal, activin A and activin B.
 3. The method of claim 2,wherein the TGFβ superfamily growth factor is activin A.
 4. The methodof claim 3, wherein at least 10 ng/ml activin A is provided.
 5. Themethod of claim 3, wherein at least 100 ng/ml activin A is provided. 6.The method of claim 1, wherein the human pluripotent stem cells comprisehuman embryonic stem cells.
 7. The method of claim 1, wherein the humanpluripotent stem cells are derived from a tissue selected from the groupconsisting of the morula, the ICM of an embryo and the gonadal ridges ofan embryo.
 8. The method of claim 1 further comprising the step ofproviding serum to the human pluripotent stem cells.
 9. The method ofclaim 1 further comprising a medium that comprises less than about 2%(v/v) serum.
 10. The method of claim 1, wherein the population ofmesendoderm cells express at least one marker selected from the groupconsisting of brachyury, FGF4 or SNAI1 and at least one marker from thegroup consisting of OCT4, SOX17, CXCR4, FOXA2, SOX7 or SOX1.
 11. Themethod of claim 10, wherein the expression of the marker selected fromthe group consisting of brachyury, FGF4 and/or SNAI1 is greater than theexpression of the marker selected from the group consisting of OCT4,SOX17, CXCR4, FOXA2, SOX7 and/or SOX1 in the population of mesendodermcells.
 12. The method of claim 1, wherein at least about 15% of thepluripotent human stem cells differentiate into mesendoderm cells. 13.The method of claim 1, wherein at least about 50% of the pluripotenthuman stem cells differentiate into mesendoderm cells.
 14. The method ofclaim 1, wherein at least about 90% of the pluripotent human stem cellsdifferentiate into mesendoderm cells.
 15. The method of claim 1, whereinthe said pluripotent human stem cells are further provided with one ormore growth factors selected from the group consisting of FGF10, FGF4,FGF2 or Wnt3B.
 16. An in vitro method of producing human mesendodermcells, the method comprising: obtaining a cell population comprisinghuman pluripotent stem cells; and providing said cell population with aTGFβ superfamily growth factor, thereby generating a cell populationcomprising at least 10% mesendoderm cells.
 17. The method of claim 16,further comprising allowing sufficient time for the population ofmesendoderm cells to form.
 18. The method of claim 1, further comprisinga medium that comprises no serum.
 19. The method of claim 18, furthercomprising a medium that comprises no serum replacement.
 20. The methodof claim 2, further comprising a medium that comprises no serum.